Nanoscopic aggregation in random copolymers of methyl methacrylate (MMA) with n-butyl methacrylate (n-BMA) or benzyl methacrylate (BzMA) in bulk films has been studied for the first time at low temperatures by using photochemical hole burning (PHB) spectroscopy for tetraphenylporphine (TPP) dispersed in the copolymers and also with transmission electron microscopy (TEM). Temperature cycle experiments with PHB were carried out for the copolymers with different ratios of comonomer groups. The increases in hole widths due to structural relaxation of butyl groups or benzyl groups in the copolymers of n-BMA or BzMA with MMA were much larger than those expected for hypothetical systems where MMA and comonomer groups with the corresponding molar ratios are randomly dispersed. The results have been attributed to the nanoscopic aggregation of n-BMA or BzMA units with the preferential inclusion of TPP within them in the films. PHB spectroscopy provides a new tool for studying nanoscopic aggregation larger than 2.3 nm in transparent random copolymer bulk films. The nanoscopic aggregation of MMA-rich domains with the diameter of 20-30 nm in a random copolymer bulk film of MMA with BzMA has also been ascertained with TEM measurements. A TEM picture of a polymer blend film of PMMA and PBzM.A (1:1) showed phase separation with PBzMA-rich domains of ca. 500 nm diameter.