Abstract The kinetic behaviour of intrinsic factor-vitamin B 12 binding has been examined under varying conditions using an albuminised charcoal separation technique. The overall reaction obeys second order rate laws. The intrinsic factor considered alone obeys first order laws; the velocity of reaction of vitamin B 12 is too fast for measurement by the technique described but by deduction obeys first order laws. Rate constants as three temperatures, ( k 2 at 25° C=1.56·10 8·mole −1·s −1) the activation energy ( E=12.7 kJ·mole −1) and Arrhenius constant ( A=2.7·10 10 1·mole −1·s −1 have been calculated. There is the possibility of diffusion control of the reaction in which case the E and A values are invalid. The effect of pH on the reaction has been studied and the results discussed in relation to the pH studies of other workers whose results show disagreement. Albumin coated charcoal was shown to discriminate between intrinsic factor-vitamin B 12 and free vitamn B 12 over a wide pH range. The apparent under-estimation of intrinsic factor in dilute solution was shown to be due to adsorption of the intrinsic factor to plastic tubes.