A novel technique using monotonic interpolation to blend and cut distillation temperatures and evaporations for petroleum fuels in an optimization environment' is proposed. Blending distillation temperatures are well-known in simulations whereby cumulative evaporations at specific temperatures are mixed together, these data points are used in piece wise cubic spline interpolations to revert back to the distillation temperatures. Our method replaces the splines with monotonic splines to eliminate the well-known oscillation effect called Runge's phenomenon, and to allow the distillation curve itself to be adjusted by optimizing its initial and final boiling points, known as cutpoints. By optimizing both the recipes of the blended material and its blending component distillation curves, very significant benefits can be achieved, especially given the global push toward ultralow sulfur fuels (ULSF), because of the increase in natural gas plays, reducing the demand for other oil distillates. Four examples are provided to highlight and demonstrate the technique, where we have good agreement between the predicted actual evaporation curves of the blends.