A new, software controlled electrochemical measuring system for the simultaneous measurements of surface mass changes and optical absorbance spectra has been developed. A usual EQCN apparatus with transparent ITO covered quartz crystal working electrode was built in a three-electrode cell made of polyvinylidene fluoride. A detailed description of both the apparatus and the software are given in the paper. The system was tested by the deposition and electrochemical transformation of polyaniline films. A new insight into the properties of the system is given by introducing the molar absorptivity based on simultaneous mass an absorption measurements. (C) 2014 Elsevier B.V. All rights reserved.