An improved form of Rice–Ramsperger–Kassel (RRK) theory in which account is taken of the way in which the various normal-mode vibrations and rotations contribute to reaction, and allowance is made for the zero-point energies. In this theory the energy ɛ* in an energized molecule is classified as either active, ɛ*active, or inactive, ɛ*inactive. The rate depends upon P ɛ *active N ɛ *, where N ɛ * is the density of states having energy between ɛ* and ɛ* + dɛ*, and P ɛ *active is the sum of the active quantum states of the activated complex. This extension of RRK theory brings it in line with transition-state theory.