Abstract: The effect of acidity of sample solution introduced into the atomizer after addition of the mixed solution containing KBH4 in the HG-AFS determination of As,Cd,Pb,Hg and Se were studied,and optimized conditions for their determinations were given.It was shown that the acidity of the solution was most critical for determination of lead,which should be controlled in a narrow range of cHNO3 0.20-0.22 mol·L-1.For the determination of cadmium,broader range of acidity between cHCl 0.20-0.31 mol·L-1 were tolerable.While for the determination of arsenic,selenium and mercury,the fluoresence intensity were very slightly effected by the acidity of test solution,and satisfactory results were obtained under the acidity of cHCl>0.1 mol·L-1 (for As),cHCl>0.12 mol·L-1 (for Se) and cHNO3>0.16 mol·L-1 (for Hg).