Peroxy radical hydrogen shift (H-shift) reactions are key to the formation of highly oxidized organic molecules and particle growth in the atmosphere. In an H-shift reaction, a hydrogen atom is transferred to the peroxy radical from within the same molecule to form a hydroperoxy alkyl radical, which can undergo O 2 uptake and further H-shift reactions. Here we use an experimentally verified theoretical approach based on multi-conformer transition state theory to calculate rate constants for a systematic set of H-shifts. Our results show that substitution at the carbon, from which the hydrogen is abstracted, with OH, OOH, and OCH 3 substituents lead to increases in the rate constant by factors of 50 or more. Reactions with CO and CC substituents lead to resonance stabilized carbon radicals and have rate constants that increase by more than a factor of 400. In addition, our results show that reactions leading to secondary carbon radicals (alkyl substituent) are 100 times faster than those leading to primary carbon radicals, and those leading to tertiary carbon radicals a factor of 30 faster than those leading to secondary carbon radicals. When the carbon from which the H is abstracted is secondary and has an OH, OOH, OCH 3 , CO, or CC substituent, H-shift rate constants are larger than 0.01 s −1 and need to be considered in most atmospheric conditions. H-shift reaction rate constants are largest and can reach 1 s −1 when the ring size in the transition state is 6, 7, or 8 atoms (1,5, 1,6, or 1,7 H-shift). Thus, H-shift reactions are likely much more prevalent in the atmosphere than previously considered.