Pentadienyl radicals were generated from penta-1,4-diene and cis and trans-1-bromopenta-2,4-diene, and were observed in the E,E (1) and E,Z (2) conformations in hydrocarbon solution by e.s.r. spectroscopy. The E,Z-radicals are converted into the E,E-radicals at T > ca. 170 K, but the E,E-radicals are not converted into the E,Z-radicals in the accessible temperature range. From the estimated barrier to rotation in E,Z-pentadienyl radicals the methane based stabilization energy (E8Me-H) was estimated to be 104 kJ mol-1. Pentadienyl radicals can also be obtained from ring-opening of cyclobut-2-enylmethyl radicals (3). Bromine abstraction from cyclobut-2-enyl methyl bromide by triethylsilyl radicals gave cyclobut-2-enylmethyl radicals at temperatures below ca. 230 K. Above this temperature homolytic fission of the cyclobutene ring occurred and pentadienyl radicals in the E,E-conformation were detected by e.s.r. Initially, E,Z-pentadienyl radicals must be formed, but at the temperature of ring fission these are converted into the E,E-radicals and so are not observed. Hydrogen abstraction from neither 3-methylcyclobutene nor from bicyclo[2.1.0]pentane yields (3): instead 3-methylcyclobutenyl radicals and cyclopent-3-enyl radicals are formed, respectively. E.s.r. parameters are also reported for a range of substituted pentadienyl radicals generated from the corresponding 1,4-dienes. Of these radicals only 3-trimethylsiloxypentadienyl was observed in two conformations.