In contrast to the usual behaviour of mixed cation glasses (large deviations from additivity with a pronounced minimum at the conductivity isotherms and a diffusivity crossover when guest cations reduce the mobility of ions coming from the host material), the CuI-AgI-As2Se3 glassy system exhibits remarkable differences in the transport properties. First, the ionic conductivity increases monotonically by four orders of magnitude from 3 X 10(-8) to 5 X 10(-4) S cm(-1) when CuI is gradually replaced by AgI, without any minimum at intermediate concentrations. Secondly, no diffusivity crossover was observed. The Ag-110 tracer diffusion coefficient D-Ag follows the conductivity isotherm with nearly the same changes in D-Ag of four orders of magnitude, from approximate to 10(-12) cm(2) s(-1) to approximate to 10(-8) cm(2) s(-1). The Cu-64 tracer diffusion coefficient D-Cu increases by a factor of 20 to 30 when copper is substituted by silver. Moreover, the values of D-Cu are always lower than D-Ag. The only exception is a 0.5CuI . 0.5As(2)Se(3) glass, for which D-Ag and D-Cu are very similar. The results obtained can be described as a degenerated mixed cation effect with different trends for silver and copper mobile cations.