It's often a problem that the measurement of reaction rate constant is influenced by the clustering of foreign molecule to one of the reactant partners especially in Atmospheric Physics and Physical Chemistry. The presence of such clustering also hinders to extract sufficient information pertaining to a reaction like Gibb's free energy which is important thermodynamic variable. Even though the problem is inevitable in such areas, no much and sound work is done to understand it. The task of this book is then devoted to examine such effects particularly focused on water clustering effect on reaction rate constants and their corresponding thermodynamic variables. A computer simulation was made to compute this effect. The computation was based on the Average Dipole Orientation and the Hard Sphere Average Dipole Orientation theories. The reactions considered were the NI-PTRs. Such reactions are of great importance in enormous field of studies, environmental science to mention the most common one. These computed results facilitate the use of NI-PTR Mass Spectrometry technique in the application of laboratory investigation and field research of environmentally important VOCs.