The constant reduction in critical dimensions required for new device generations and the use of sub-wavelength lithography impact the quality of the aerial image transferred into the resist layer. As an example, the intrinsic bias between isolated and dense features in the aerial image is becoming more and more significant, requiring better performance from the resist process to cancel this effect. This work investigates the process mechanisms leading to Iso-Dense bias (I-D bias) reduction for two 193 nm methacrylate based resists at constant optical settings, as a function of PEB temperature. In both cases, it has been possible to find optimized process conditions leading to reduced I-D bias values, but it appears that the leading mechanisms involved during PEB are different and do not seem equivalent in terms of resist capabilities. Reaction controlled resists, which work with a Diffusion Well effect during PEB, that is a high diffusion contrast between exposed and unexposed areas, allow I-D bias compensation without degrading resolution performance. On the contrary, diffusion controlled resists, which usually require high Post Exposure Bake (PEB) temperature to thermally boost the deprotection reaction, do not keep a high diffusion contrast between exposed and unexposed areas during PEB. Consequently, for these resists, best process conditions for I-D bias reduction do not correspond to the optimized process conditions for other resist performance, such as resolution and DOF. In this paper, the two different mechanisms which drives the acid catalyzed deprotection during the Post Exposure Bake step have been studied using different characterization techniques (modulated Temperature DSC, Dielectric analysis, in-situ Ellipsometry) and process performance has been correlated with 193 nm resist component properties (Polymer matrix, protecting groups or PAG characteristics).