A novel, low-cost, rapid, accurate, non-invasive and high throughput method based on the principles of Optical Interferometry (OPTI method) has been developed and applied for the in situ monitoring in one simple run of first (melting) and second (glass transition) order transitions as well as of the thermally induced decomposition of various thin polymeric films spin coated on flat reflective substrates (untreated silicon wafers). The new method has been applied successfully for measuring the glass transition, melting and decomposition temperatures of six commercially available polymers [poly(methyl methacrylate) (PMMA), poly(2-hydroxyethyl methacrylate), (PHEMA), poly(vinyl acetate-co-crotonic acid), (PVACA), poly(vinyl pyrrolidone) (PVP), polyvinyl chloride-co-vinyl acetate) (PVCVA) and crystalline poly(vinylidene fluoride-co-hexafluoropropylene) (PVFHP)] of known T(g)s or T(m)s. The recorded interferometric signals were identified and characteristic signal patterns were qualitatively correlated to specific transitions. The monitoring of first and second order transitions in thin polymeric films is based on detectable differentiations of the total energy of a fixed wavelength laser beam incident almost vertically (angle of incidence <5degrees) onto a thin polymeric film spin coated on a flat reflective substrate. These differentiations are caused by film thickness and/or refractive index changes of the polymeric film both resulted from the significant change of the polymer's free volume taking place on the transitions. For film thicknesses over approx. 200-250 nm, the T-g or T-m of the polymeric films measured with the OPTI method were in excellent agreement with the corresponding values of the polymer, measured by DSC. An investigation on the trends of the T-g of PHEMA and PMMA films in a wide thickness range (30-1735 nm) was also carried out. Ultra-thin (similar to 30 nm) films of PMMA and PHEMA showed significant increase in their T-g values by approx. 30degreesC upon comparing to their corresponding bulk T(g)s. This behavior was attributed to an enhanced polymer-surface interaction through hydrogen bonding and/or to changes in the tacticity of the polymer.