The total cross sections for collisions between excited potassium and unexcited rubidium atoms, leading to the transfer of excitation between the 42P states in potassium, have been determined in a sensitized fluorescence experiment. The experiments were carried out at partial pressures of potassium vapor lower than 10−5 mm Hg, at which the imprisonment of resonance radiation may be disregarded. The cross sections Q12″ (42P1/2 → 42P3/2) and Q21″ (42P1/2 ← 42P3/2) equal 260 Å2 and 175 Å2, respectively, and are in the ratio predicted by the principle of detailed balancing.