DUV positive acting chemically amplified resists are imaged by exposure to light, to generate acid, followed by exposure to heat, to activate acid catalytic deprotection of the resin. There are four mechanisms for imaging the photoresist polymer. The mechanism that dominates depends on the temperature range being examined. The lowest temperature region is effectively non-catalytic. The next three regions are catalytic. The lithography for each one of these regions is unique. In the lowest temperature catalytic region, the isolated line features require more exposure than grouped lines to attain target sizing. In the next higher temperature region, the relative sizing for the two types of lines are reversed. The highest temperature region is a result of thermal degradation of the polymer. This paper reviews experimental isolated line to grouped line bias dependence on PEB temperature for an acid catalyzed resist; and then, using experimentally determined modeling parameters, and the lithographic simulator, PROLITH/2, interprets this phenomena to be a result of the degree of acid diffusion within each catalytic region.