This paper is aimed at investigating the removal of both easily as well as slowly biodegradable (or rate-limiting) organic substances in an aerobic hybrid reactor which consists of both the suspended and attached biomass (or biofilm). The study involves theoretical computer simulations on the reactor performance. It has been shown that the organic removal efficiency in a purely suspended growth system decreases markedly when the maximum specific removal rate (MSRR) of an organic substance is lower than 0.5/day. Thus, a suspended growth system is not effective in removing any substance of low biodegradability. On the contrary, when a hybrid system is applied, a rate-limiting organic substance with an MSRR of 0.1/day can still be effectively removed as long as the weight ratio of the biofilm to the suspended biomass is not less than 0.027. In such a system, an increase in the amount of the suspended biomass will not significantly increase the removal rate for such a substance because a slowly biodegradable substance needs some specific types of microorganisms which have a low growth rate. As such, in a biofilm system it is easier to maintain these microorganisms than in a suspended growth reactor. In this case, an increase of biofilm will improve the treatment efficiency. For the removal of an easily biodegradable substance (i.e., MSRR is not less than 0.5/day), the suspended biomass plays a more important role than the fixed biomass in a hybrid reactor. Through simulation modelling, it is known that to obtain 80% or more removal of a slowly biodegradable substance (MSRR at 0.1/day) in a hybrid reactor, the optimal operating values of organic loading rate, hydraulic retention time, and the ratio of support surface to reactor volume are 0.53 kg COD/m3/day, 6 hours, and 225 m2/m3, respectively. The results of this study suggest that to achieve an effective simultaneous removal of both easily and slowly biodegradable organics a hybrid reactor offers the best solution.