This paper describes the file formats of the output data and geometry files generated by the Delphin program, a simulation model for hygrothermal transport in porous media. The output data format is suitable for any kind of simulation output generated by transient transport simulation models. Implementing support for the Delphin output format enables use of the advanced post-processing functionality provided by the Delphin post- processing tool and its dedicated physical analysis functionality. The article also discusses the application programming interface of the DataIO library that can be used to read/write Delphin output data and geometry files conveniently and efficiently.:1 Introduction 2 Magic Headers 2.1 Version Number Encoding in Binary Files 2.2 Version Number Encoding in ASCII Files 3 Output Data File Format 3.1 File Name Specification 3.2 General File Layout 3.3 ASCII Format 3.4 Binary File Format 4 Geometry File 4.1 General 4.2 File Name Specification 4.3 ASCII Format 4.4 Binary Format 5 Overview of the Programming Interface (API) 5.1 Reading Data Files with the DataIO Library 5.2 Writing Data Files with the DataIO Library 6 Summary