Abstract: Aformula for the calculation of the equivalent volume of acid mixture titration is derived. Within this formula the measured value of the hydrogen - iois expressed by activity. Anonlinear model parameter estimation methodmaximum likelihood estimation method (MLE) is presented. It takes into account the different errors in pHand V (vol.) data obtained from the determination of the mixture of acids. It gives out more satisfactory results than that obtained from the least squares estimation method (LS). The Monte Carlo method has been employed to the study of the results obtained from methods of MLEand LS.