Summary The CO 2 compensation concentration of detached leaves of Moricandia spinosa and Moricandia moricandioides was measured by means of infrared gas analysis. At 21 % O 2 the mean value for the CO 2 compensation concentration (I') was 24μl·1 −1 CO 2 in M. spinosa and 58 μl·1 −1 CO 2 in M. moridandioides . The O 2 dependence of T in M. spinosa was nonlinear below 21 % and linear between 21 and 50% oxygen. In M. moricandioides this relationship is linear between zero and 50% O 2 . The rate of photorespiration in M. spinosa is about four times lower than in M. moricandioides . The data suggest that M. spinosa is a species, intermediate in C 3 , C 4 photosynthesis, whereas M. moricandioides is a typical C 3 plant.