Kinetic estimation of chemical reactions is considered to be a crucial step prior to design of a robust, controllable, and safe production process. Spectroscopic measurements are widely used for kinetic analysis. However, there may be unwanted contributions in the measured spectra, which may come from the instrumental variations (such as the baseline shift or distortion) or from the presence of inert absorbing interferences with no kinetic behavior. Here, the kinetic estimation problem with time-invariant contributions is studied in depth, and we derive conditions where the estimation accuracy is not affected by the time-invariant contributions. Moreover, kinetic parameter estimation and separation of time-invariant contributions can be performed simultaneously under proper conditions. Also, an approach for kinetic parameter estimation based on spectra with time-variant contributions is proposed. Finally, a novel unified framework is developed for kinetic parameter estimation when there is no prior information for unwanted contributions.