The etch rates and plane selectivity for single-crystal silicon anisotropic etching in aqueous rubidium hydroxide are reported. Silicon wafers of (100) and (110) orientation were etched in 25, 30, 40, and 50 weight percent (w/o) aqueous RbOH at 50, 60, 70, and 80-degrees-C. The activation energy, based on an Arrhenius equation, was 0.48 eV for the (100) and (110) planes. The etch rate ratio for the (110)/(100) planes was equal to 1.5 at 50 w/o, and 0.6 at 25 w/o. The plane selectivity is not a function of temperature. Silicon spheres, approximately 0.25 in. diam were etched to reveal fast etching high index {522}/{311} planes in the vicinity of the [100] direction.