Abstract Membrane-bound multi-protein complexes in mitochondria are provisionally classified into four categories based on possible mechanism of their assembly and degradation. These mechanisms may be investigated by the use of pulse-labeled radioactive markers which are not re-utilizable. Age dependent assembly is defined as that mechanism by which one or more of the pulse-labeled subunits are assembled into a complex, only while this complex is assembled. If the labeled sub-units can be taken up by the complex randomly during its life-span, then the mechanism is called age-independent assembly. Age-dependent degradation was defined as that mechanism by which the labeled subunits are decomposed, only when the complex is being degraded as an entity. If the labeled subunits are decomposed randomly, the mechanism is called age-independent degradation. Four categories are made by combining each of the assembly and degradation mechanisms. A differential equation was obtained to describe the fate of labeled sub-units that follow the age-dependent assembly and age-dependent degradation. Also derived was an equation for the age-independent assembly and age-dependent degradation. The other two categories which involve the age-independent degradation after age-dependent or age-independent assembly are described by single exponential kinetics. Practical application of the equations is illustrated with the use of experimental data on mitochondrial turnover found in the literature which suggests that the pulse-labeled proteins in rat liver mitochondria may follow the age-dependent assembly and degradation. The present attempts to introduce the concept of ageing into multi-protein complexes in mitochondria are the extensions of the steady state theory of mutation by Eyring & Stover (1970).