This study uses longitudinal panel data and short-term retest data from the same respondents in the German Socio-economic Panel to estimate the contribution of state and trait variance to the reliable variance in judgments of life satisfaction and domain satisfaction. The key finding is that state and trait variance contribute approximately equally to the reliable variance in well being measures. Most of the occasion specific variance is random measurement error, although occasion-specific variation in state variance makes a reliable contribution for some measures. Moreover, the study shows high similarity in life satisfaction and average domain satisfaction for the stable trait component (r = .97), indicating that these two measures are influenced by the same stable dispositions. In contrast, state variance of the two measures is distinct, although still highly correlated (r = .77). Error variances of the two measures are only weakly correlated, indicating that most of the error component is indeed due to random measurement error.