Context. Interstellar grains are known to be important actors in the formation of interstellar molecules such as H2, water, ammonia, and methanol. It has been suggested that the so-called interstellar complex organic molecules (iCOMs) are also formed on the interstellar grain icy surfaces by the combination of radicals via reactions assumed to have an efficiency equal to unity.Aims. In this work, we aim to investigate the robustness or weakness of this assumption. In particular, we consider the case of acetaldehyde (CH3CHO), one of the most abundant and commonly identified iCOMs, as a starting study case. In the literature, it has been postulated that acetaldehyde is formed on the icy surfaces via the combination of HCO and CH3. Here we report new theoretical computations on the efficiency of its formation.Methods. To this end, we coupled quantum chemical calculations of the energetics and kinetics of the reaction CH3 + HCO, which can lead to the formation of CH3CHO or CO + CH4. Specifically, we combined reaction kinetics computed with the Rice-Ramsperger–Kassel–Marcus theory (tunneling included) method with diffusion and desorption competitive channels. We provide the results of our computations in the format used by astrochemical models to facilitate their exploitation.Results. Our new computations indicate that the efficiency of acetaldehyde formation on the icy surfaces is a complex function of the temperature and, more importantly, of the assumed diffusion over binding energy ratio f of the CH3 radical. If the ratio f is ≥0.4, the efficiency is equal to unity in the range where the reaction can occur, namely between 12 and 30 K. However, if f is smaller, the efficiency dramatically crashes: with f = 0.3, it is at most 0.01. In addition, the formation of acetaldehyde is always in competition with that of CO + CH4.Conclusions. Given the poor understanding of the diffusion over binding energy ratio f and the dramatic effect it has on the formation, or not, of acetaldehyde via the combination of HCO and CH3 on icy surfaces, model predictions based on the formation efficiency equal to one should to be taken with precaution. The latest measurements of f suggest f = 0.3 and, if confirmed for CH3, this would rule out the formation of acetaldehyde on the interstellar icy surfaces. We recall the alternative possibility, which was recently reviewed, that acetaldehyde could be synthesized in the gas phase starting from ethanol. Finally, our computations show the paramount importance played by the micro-physics involved in the interstellar surface chemistry and call for extensive similar studies on different systems believed to form iCOMs on the interstellar icy surfaces.