The hardness of Al-5wt%Zn (alloy A) and Al-5wt%Zn-0.25wt%In (alloy B) was measured at room temperature for samples heat treated in the range 300-453 K and dwell times in the range 30-300 s under 50 gm load. Softening was observed for all the samples and the hardness decreased with increasing temperature and/or dwell time. Hardness drop was larger for alloy (B), which in general showed higher hardness than alloy (A). The stress exponent n increased with increasing temperature and showed high values falling in the power law breakdown region. The parameters deduced from the analysis of X-rays data and micrographs were found to be consistant with the calculated mechanical data.