The influence of the method for modifying high-silica ZSM-5 zeolites by lithium on their acid and catalytic properties was studied by measuring the thermodesorption of ammonia and the catalytic conversion of C3–C4 alkanes. Modifying zeolites of this type by lithium was found to decrease their activity as a result of predominantly poisoning Broensted acid centers, to considerably increase selectivity with respect to ethylene and propylene, and to decrease the yield of arenes and the formation of coke deposits. The most effective method for the introduction of lithium into zeolites was mechanical mixing; the optimal amount of introduced lithium equaled 3%.