For the first time optically detected cyclotron resonance (ODCR) has been demonstrated using a CO2 pumped far-infrared (FIR) laser instead of microwaves. Both the electron and the light-hole cyclotron resonances have been observed in GaAs, as well as the 1 s to 2p+ impurity transitions. Valence band quantum effects, well known in Ge, are resolved directly for the first time in GaAs and the electron cyclotron resonances show strong spin doublet in the highest quality MBE samples. The technique has remarkable resolution and sensitivity at low temperatures and, by contrast with other techniques that have been reported, the authors also observe the n=1 to 2 (polaron shifted) and higher spin doublet split resonances at helium temperatures and with low FIR laser power. The conduction band results are analysed on the five-band model and the implications of this model on the valence band results are discussed. The authors have determined the valence band inverse mass parameters to be: gamma 1=7.5, gamma 2=2.6, gamma 3=3.1, kappa =1.0.