ABSTRACT The optimum injection rate for the matrix acidizing treatments of carbonate reservoirs has been the subject of debate and several recent experimental studies. Because acid attacks the rock in different modes depending on the injection rate (i.e., compact dissolution, dominant wormhole, leakoff-dominated wormholing and uniform dissolution), it is essential to identify the rate at which the transition between dominant wormholes and leakoff-dominated wormholing occurs. This is the optimum injection rate at which the greatest penetration is accomplished with the least volume of acid injected. Linear or radial laboratory experiments can be done from which field conditions can be extrapolated through dimensional analysis. This is the subject of this paper. Optimum HCl injection rates for a low- and a moderate-permeability limestone using radial cores have been determined. The effects of temperature and acid concentration have also been studied. Analysis of the results shows that the optimum injection rate depends greatly on the rock and reservoir conditions and can be determined from experiments.