Polymethacrylonitrile (PMCN) and polystyrene (PSt) were irradiated in the absence of O 2 with heavy particle radiations 275 MeV 20 10Ne 7+ ions and 180 MeV 40 18Ar 8+ ions. With PMCN the 100 eV-yield of main-chain scissions was found to decrease with increasing LET (in MeV/μm): G(S) = 3.3–3.6 at dE/dx= 2 × 10 -4 ( 60Co-γ-rays), G(S) = 0.5 at dE/dx = 0.43 (Ne 7+ ions) and G(S>) = 0.4 at dE/dx = 2.5 (Ar 8+ ions). With PSt the 100 eV yield of crosslinks was found to be almost independent of LET: G(X) = 0.05 at dE/dx = 2 × 10 -4 ( 60Co-γ-rays), G(X) = 0.05 at dE/dx = 0.37 (Ne 7+ ions) and G( X) = 0.03 at dE/ dx = 2.3 ( Ar 8+ ions). These results are critically discussed with respect to 10 times higher G( X) values at high LET reported by other authors for PSt (Calcagno et al., 1987; Puglisi et al., 1987). Poly-2-chloroethylmethacrylate (PMCMA) turned out to be a probe material for evidencing pronounced temperature increases in the polymer sample during irradiation at high LET; PMCMA was crosslinked to an insoluble gel, if not cooled carefully during irradiation, whereas it underwent predominant main-chain cleavage if sufficiently cooled.