The karyotypes of three species of the genus Thinouia (Sapindaceae, Paullinieae) from southern Brazil and Bolivia are analyzed. New counts and karyotypes are reported for the first time for T. ventricosa and T. paraguayensis, both with 2n = 28. The karyotype formulae of T. mucronata, T. paraguayensis and T. ventricosa are 8m 14sm 6st, 10m 8sm 10st, and 14m 4sm 10st, respectively. In all the species investigated, the prophase chromosome condensation was always proximal and the pattern of the interphase nuclei was non-reticulate. These results are discussed in relation to previous chromosome studies for Paullinieae in order to infer the phylogenetic relationships within the tribe.