Conventional well test analyses typically assume that the wellbore skin factor is constant over the entire duration of the well test. For certain cases such as during acidizing treatments, however, the skin factor can change significantly and affect the pressure response. The concept of well test analysis has been used in real-time evaluation of well stimulation treatments. Numerical and analytical methods have been developed to estimate the evolution of skin during acidizing, but these methods can be time consuming and cumbersome when numerous pressure and rate data exist. In this paper, we apply the flow rate normalization method used in well testing to estimate skin as a function of time. We demonstrate that the new technique is accurate, robust, fast, and easy to use. We apply the method to two matrix acidizing treatments performed in a water well and an oil well. The skin calculations for each example agree well with those calculated from current accepted methods. The new technique is a fast, simple and more accurate method for real-time interpretation of acidizing treatments where a multitude of pressure and flow rate data exists. We compare the results from the new method to current methods for several field examples.