The kinetics of coupled heterogeneous catalytic reactions with a common reagent (competitive reactions) are considered. A simple method to determine relative reactivities by competitive experiments is presented. Noteworthy, this method may be used without knowledge of the complete form of the rate equation if only a mechanism of the initial interaction of the competitive reagents with the catalyst surface is the same. A similar approach is developed for partial oxidation reactions and other consecutive or parallel-consecutive ones. By analysis of the kinetics of competitive reactions of a Langmuir-Hinshelwood type it is shown that at certain values of the kinetic parameters one competitive reaction can be accelerated by another. Another non-trivial effect of mutual influence of competitive reactions consists in a sharp inhibition of one reaction by another, even if the surface coverage by adsorbed species is negligible. This phenomenon is inherent in reactions that involve the reversible dissociative adsorption of a heteroatomic reagent. To confirm the obtained theoretical results, the experimental kinetic data for CO-NO-O2, CO-H-2-O2, CH4-O2-H-2 and some other systems are examined.