The chemoionization processes in Rydberg - Rydberg and Rydberg - ground-state rubidium collisions at 60 K were investigated for the principal quantum number n = 11. The ratio of the rate constant k₂ for atomic ion formation in Rydberg - Rydberg collisions to the rate constant k₁ for associative ionization in Rydberg - ground-state atomic collisions was measured. The value k₂ = (1.5 ± 0.4) × 10^-8 cm³ s^-1 was determined using the literature data for k₁. The value obtained coincides to a factor of two with that calculated within the Katsuura-Smirnov model.