Composite layers of poly(3,4-ethylenedioxythiophene) and diamond nanoparticles were prepared by an electrocodeposition procedure from an electrolyte containing the monomer 3,4-ethylenedioxythiophene and a suspension of diamond nanoparticles in acetonitrile/water mixtures. For the in situ monitoring of the deposition process we used piezoelectric quartz crystals with gold electrodes as substrates (electrochemical quartz crystal microbalance, EQCM). The inclusion of the diamond particles increases the stiffness of the layers. This effect can be directly quantified via the electroacoustic impedance of the quartz crystal. We use a mean field model for the interpretation of our EQCM data which yields the complex shear modulus of the composite layers. The volume fraction of diamond (as determined by ex situ XPS) enters as a parameter in the evaluation procedure. Finally, we could estimate the solvent content of the composite layers and the shear moduli of the pure dry PEDOT matrix.