For grafted copolyimides with variable length and grafting density of polymethylmethacrylate side chains, the molecular masses were determined using three modes of SEC analysis, which differ in the detectors set. The results were compared with the values obtained (i) using ¹H NMR spectra, (ii) those calculated by summing the molecular masses of homopolymer linear blocks that make up the copolymer, as well as (iii) with the absolute values of molecular masses determined by sedimentation-diffusion analysis. It is shown that for branched grafted copolyimides the use of only a refractometric detector or its combination with a viscometric detector in the SEC analysis did not allow for obtaining reliable molecular masses. On the contrary, the combination of refractometric and viscometric detectors with a light scattering detector provided for the determination of the molecular masses close to the respective absolute values. It is also shown that for linear block copolymers, unlike branched copolymers, the molecular masses close to the absolute values can be obtained using any of the three modes of SEC analysis. For the first time, a concept of the critical molecular mass of the side chains of grafted copolyimides is introduced as a quantitative criterion that takes into account the total length and also the density of grafting of the side chains.