An experimental set-up based on multiwavelength interferometry, is applied in order to study in-situ the dissolution process of thin resist films. The interference function was the basis for a fitting algorithm, which analyses the experimental data and evaluates the progress of the resist thickness with time. The dissolution of various PMMA molecular weights (15 K, 350 K, 996 K) and resist thicknesses (20-300 nm), in various developers, consisting of mixtures of methyl iso butyl ketone (MIBK), iso propanol (IPA), H 2 O at various relative concentrations, was studied. Surface dissolution inhibition was shown in unexposed resists with high molecular weights and increased thickness in the case of MIBK-IPA 1-1 and IPA-H 2 O 7-3 developers. After that the whole dissolution process evolved at a steady rate. Dissolution of thick films proved to be unpredictable showing complex dissolution curves. Low molecular weight resists presented a smooth dissolution curve without dissolution inhibition. Samples exposed with small DUV doses, exhibited dissolution behavior similar to the unexposed cases, whereas high exposure doses, within the lithographically useful range, led to smooth dissolution behavior. In the case of MIBK-IPA 1-3 developer significant swelling was observed.