In this work, for the first time, reactive radical-cation species present in hole-transporting materials were shown to react with tert-butylpyridine additive, routinely used in hole transporting layer composition. As a result, new pyridinated products were isolated and characterized by NMR and MS analysis. Additionally, their optical and photophysical properties (i.e., solid-state ionization potentials (Ip), cyclic voltammetry (CV), UV/vis characteristics, and conductivities) were determined. Formation of the pyridinated products was confirmed in the aged perovskite solar cells by means of mass spectrometry, and shown to have negative influence on the overall device performance. We believe that these findings will help improve the stability of perovskite devices by either molecular engineering of hole-transporting materials or utilization of lessreactive or sterically hindered pyridine derivatives.