CER File

What are CER files and how to open them

Are you having problems opening a CER file or are you simply curious about its contents? We're here to explain the properties of these files and provide you with software that can open or handle your CER files.

What is a CER file?

CER files have multiple uses, and Internet Security Certificate is one of them. Read more about the other uses further down the page.

Internet Security Certificate

These files pertain to Internet security certificates, which are digital documents that serve as verifiable credentials for online entities.

The certificates are issued by trusted third parties known as Certificate Authorities (CA). They are used to establish the authenticity and legitimacy of websites and encrypt communication between parties to safeguard sensitive information.

Internet security certificates are an integral component of online security protocols, and the CER file format is used to store such certificates to enable their use on various computing devices.

How to open CER files

Important: Different programs may use files with the CER file extension for different purposes, so unless you are sure which format your CER file is, you may need to try a few different programs.

While we have not verified the apps ourselves yet, our users have suggested eight different CER openers which you will find listed below.

Last updated: August 31, 2026

All known file formats using extension .CER

While Internet Security Certificate is a popular type of CER-file, we know of 2 different uses of the .CER file extension. Different software may use files with the same extension for different types of data.

MSI/Accelrys Cerius II

MSI/Accelrys Cerius II files store detailed molecular structure information for the Cerius² modeling environment. They hold data such as atomic labels, element types, bond connectivity, molecular models' hierarchy (atoms, bonds, groups), spatial coordinates, partial charges, and other properties required to fully describe a chemical model. These files are used to save and exchange all aspects of molecular data needed for simulation, visualization, or further modeling in Accelrys' Cerius² (later part of Materials Studio) software.

The format is produced and read by Cerius² itself. When working within that environment, structures are saved in this native format to preserve both geometry and rich metadata. Models created or imported into Cerius² remain editable with full lossless fidelity in this format; other software and conversion tools may read it in (sometimes read-only), but may not preserve every attribute. Open Babel, for example, supports this format for reading structured models converted from Cerius².

Files begin with a textual header such as "# MSI CERIUS2 DataModel File Version 2 0". Within the file, data is organized hierarchically in objects like Model, Atom, Bond, etc. Attributes are tagged (A), typed (C for string, I integer, F floating-point, D double-precision, O for object reference), and include things like XYZ coordinates in double precision, charge values, labels, and connectivity. Typical internal keywords include (A C Label), (A I ACL), (A D XYZ), (A F Charge), and object delimiters like (1 Model), (Atom), (Bond).

Various apps that use files with this extension

These apps are known to open certain types of CER files. Remember, different programs may use CER files for different purposes, so you may need to try out a few of them to be able to open your specific file.

Help us help others

File.org helps thousands of users every day, and we would love to hear from you if you have additional information about CER file formats, example files, or compatible programs. Please use the links below or email us at submit@file.org to discuss further.