In this paper we use administrative tabulations from occupation-based income tax (class tax) to estimate income inequality in the Duchy of Warsaw. We start off by estimating income inequality in the Department of Kalisz, and then use the decomposability of the Theil index to estimate national income inequality based on a sample of Theil indices corresponding to different settlement types. According to our results, income inequality in the Duchy was at a moderate level, although in the biggest cities it was relatively high. Income inequality at county level was positively correlated with the mean income of the county.