The continuous economic drive in microelectronics, such as computer chips, has reached a point where acid diffusion in microlithography is a limiting factor. We have devised a new technique to track acid diffusion as it occurs, rather than after it has happened like all other applicable techniques. By spin coating two polymer films, one on top of the other, and tracking the decrease in the absorbance of the neutral form of a new pH sensitive dye, DSBS (lambda max = 424nm) while the films were being heated, the diffusion of the acid into the bottom layer was tracked and its diffusion constant will be calculated.