Current oscillations have been found during codeposition of copper and tin from sulfate solutions containing the surface-active substance (SAS) Laprol 2402C in the region of negative slope in the voltammogram. These phenomena are interpreted in terms of the transition from one stable state to another. The effective rate constant for the latter state was suggested to be significantly lower due to the inhibiting adsorption of SAS. The necessity of two conditions for oscillations to occur was confirmed, ie the existence of a negative faradaic impedance (Z(F) < 0) and the fulfillment of the inequality R-s > \Z(F)\ (R-s is the external resistance including the ohmic cell resistance). The interplay between the oscillation phenomena and the phase constitution of the alloys deposited is discussed.