The effects of thiourea (TU), benzotriazole (BTA) and 4,5-dithiaoctane-1,8-disulphonic acid (DTODSA) on the deposition of copper from dilute acid sulphate solutions have been studied using potential sweep techniques. Tafel slopes and exchange current densities were determined in the presence and absence of these organic additives. TU and BTA were found to inhibit the copper deposition reaction; increases in the BTA concentration gave a systematic lowering of the exchange current density, whilst TU behaved in a less predictable manner. For BTA and TU concentrations of 10(-5) mol dm(-3), j(0) values of 0.0027 +/- 0.0001 and 0.0028 +/- 0.0002 mA cm(-2) were obtained compared to a value of 0.0083 +/- 0.0003 mAcm(-2) for the additive free acid sulphate solution. In contrast, in the presence of DTODSA, an increased exchange current of 0.043 +/- 0.0003 mA cm(-2) was observed. The presence of additives gave rise to measured Tafel slopes of - 164, -180 and -190 mV for TU, BTA and DTODSA, respectively, compared to that of -120 mV for copper sulphate alone.