Selectivity coefficients are reported for poly(vinyl chloride) membranes containing cryptands with thiourea fragments in one of the bridges (seven compounds). These membranes exhibit rubidium selectivity in the series of Group IA and IIA metal cations. The cryptand 1,3,12,14-tetraaza-6,9,17,20,25-pentaoxabicyclo[12.8.5]heptacosane-2,13-dithione is useful as the electrode-active compound in a rubidium-selective electrode. The electrode function is linear within the activity range 10(-1)-10(-4) M, and the slope is 48 +/- 1 mV per decade. Selectivity coefficients are 0.005 for Li+, 0.006 for Na+, 0.015 for K+, 0.012 for Cs+, 0.006 for NH4+, 0.0027 for Mg2+, 0.0032 for Ca2+, 0.0040 for Sr2+ and 0.0081 for Ba2+.