The mechanism of development for positive photoresists can be described using a membrane model for the thin polymer film dissolution process instead of a simple surface-dissolution model. The model is consistent with previous findings on the factors controlling the dissolution, i.e., solubility changes due to quinone-diazide photochemistry, creation of free volume due to nitrogen gas evolution, dissolution rate effects due to cation size, and surface modification by adsorption of chemicals. One of the key reactions of the membrane model, the conversion of the phenol to a phenolate on the novolac polymer, seems to have been overlooked in previous studies. This reaction must go to completion for any dissolu-tion process to occur, and it requires a pH greater than 12.5. The membrane model will be presented and specific, supporting experiments will be discussed.