The effects of hydrogen recombination poisoners (H.R.P.) such as S, P, and As on the electrochemical behaviour and the stress corrosion cracking (SCC) of Al-Zn-Mg alloys have been studied as a function of applied potential in the range from -1800 to $-400 mV_{\mbox{SHE}}$ in 3.5 wt.% NaCl solution (pH=3.5) at the temperature of 30℃. The electrochemical behaviour was examined from the polarization curves. The SCC tests were carried out by using constant load apparatus connected with linear variable differential transformer (LVDT). Initiation and propagation periods for stress corrosion (SC) crack were differentiated by measuring displacement versus time. In the presence of H.R.P., both initiation and propagation periods for SC crack decreased and have been discussed in terms of anodic and cathodic polarization curves of specimens. From these results, it is suggested that SCC of Al-Zn-Mg alloys in 3.5 wt.% NaCl solution is caused by hydrogen embrittlement mechanism. Average SC crack propagation rates were found to be $3.1×10^{-5} -2.3×10^{-4}mm/sec$ at $-1800 mV_{\mbox{SHE}}$, $6.0 × 10^{-6} -7.5×10^{-5} mm/sec$ at $-1000 mV_{\mbox{SHE}}$, and $1.8×10^{-4} -2.0×10^{-3}mm/sec$ at $-400 mV_{\mbox{SHE}}$.
