Abstract: In a reliable step by step method of controlling frequency redefinition in a cellular mobile radio system with no severe constraints on the network or on the mobile and using only existing procedures between mobiles and the network, frequency redefinition is effected time slot by time slot by freeing each time slot step by step by first eliminating any calls in said time slot, then effecting frequency redefinition of said freed time slot by assigning it the new list of frequencies, and finally making said time slot available for traffic again.