We report experimental evidence for the formation of C5-hydroperoxyaldehydes (HPALDs) from 1,6-H-shift isomerizations in peroxy radicals formed from the hydroxyl radical (OH) oxidation of 2-methyl-1,3-butadiene (isoprene). At 295 K, the isomerization rate of isoprene peroxy radicals relative to the rate of reaction of is , or k295isom 0.002 s−1. The temperature dependence of this rate was determined through experiments conducted at 295, 310 and 318 K and is well described by . The overall uncertainty in the isomerization rate (relative to ) is estimated to be 50%. Peroxy radicals from the oxidation of the fully deuterated isoprene analog isomerize at a rate 15 times slower than non-deuterated isoprene. The fraction of isoprene peroxy radicals reacting by 1,6-H-shift isomerization is estimated to be 8–11% globally, with values up to 20% in tropical regions.