Description: This record includes software compatible with MATLAB and GNU Octave for calculating marine CO2 system variables (CO2SYS.m), computing partial derivatives of calculated CO2 system variables with respect to inputs (derivnum.m), and propagating uncertainties for CO2 system calculations (errors.m). This software builds upon previously released versions of CO2SYS.m (e.g., Lewis and Wallace, 1998; van Heuven et al., 2011; Orr et al., 2018). Major changes since CO2SYS v3.2.0: In this release, an option is included in 'CO2SYS.m' to perform pressure corrections on the CO2 solubility constant (K0) and the fugacity factor (FugFac). Previous versions of CO2SYS have evaluated these terms only at atmospheric pressure. The corrections are performed according to Weiss (1974) as described in Orr and Epitalon (2015). By default, the corrections are not made (for consistency with previous CO2SYS versions) but can be activated with the optional input pair: 'co2_press',1. PyCO2SYS has also implemented these optional corrections in v1.8.2, with differences from this version of less than 10−4%.