[en] The rate coefficients for mixing between He(2 ³P_J,MJ) levels during collisions with ground-state helium atoms and for conversion of He(2 ³P_J) atoms to He₂(b ³Π_g) molecules via three-body reactions in helium gas have been investigated over the temperature range 1.6--300 K. The measured rate coefficients for collisionally induced P-state mixing decrease slowly with decreasing temperature, from (1.8±0.5)x10^-9 cm³ s^-1 at 300 K to (4.5±0.5)x10^-10 cm³ s^-1 at 4.2 K. The rate coefficients for the production of He₂(b ³Π_g) molecules via three-body reactions are observed to increase with decreasing temperature and are described by the relation k_P congruent(2.5+267T^-1)x10^-32 cm⁶ s^-1. This behavior, which is very different from that noted in earlier studies of the conversion of He(2 ³S₁) atoms to He₂(a ³Σ_u⁺) molecules through three-body reactions, suggests that the reaction is not thermally activated