The extrapolation problems involved in predictive kinetic analysis or in determinations using slow lon-selective potentiometrlc electrodes have been studied. A simple recursive algorithm which performs the extrapolation, in cases where the mathematical function is not known, has been suggested and evaluated. The algorithm is best for extrapolateing rational functions; however, it produces reasonable results with nonrational functions as well. This method can be applied to the extrapolation of any instrumental response used in analytical chemistry. The expected error in the determinations has been studied as a function of the experimental noise level, the deviation from rational generating functions, the time domain of the measurements, and other parameters.