Method and apparatus for determining the exposed surface area of a plurality of samples of selected mineral aggregate particles which have previously been coated with asphalt and subsequently partially stripped of asphalt. The method involves placing a first sample of aggregate particles of selected size and total number into a closed chamber and subjecting the samples to a reagent solution capable of dissolving or etching certain mineral constituents of the samples, in the area which has been stripped of asphalt. Using a selected volume and concentration of such reagent solution, the pressure of the gas formed as one of the byproducts is measured as a function of time. The rate of increase of pressure will be a function not only of the reagent and its concentration, but also of the exposed area of the mineral particles. The same procedure is followed for a sample which has substantially the same average size and number of particles and presumably substantially the same total surface area, which would represent the situation for complete stripping of the asphalt coating. Comparison of the pressures at selected times will give a number representing the percentage of surface area stripped in the first procedure. In addition to measurement of pressure of a gaseous byproduct, it is possible also to measure temperature of the solution, or temperature in the gas space in the container as a function of time.