A simple method for obtaining phase transition temperatures is proposed. It is based on absolute variations of a baseline in a temperature-dependent mid-infrared transmittance spectral data set recorded for a sample KBr pellet. The method is rapid, inexpensive, and completely free of any personal bias. The method efficiency was corroborated by measuring the phase transitions of mesomorphic (trans-4-heptyl cyclohexanecarboxylic acid) and polymorphic compounds (polycyclic aromatic hydrocarbons and phenylacetylenes). In addition, the exothermic decomposition of phenylacetylenes was also monitored, thereby opening the possibility for simple and rapid monitoring of chemical reactions in solid and liquid phase. A great prospect of the method as an economical way of establishing critical temperature regions for analytical and quality control purposes is foreseen.