A site-specific Structure-Activity Relationship for the addition of OH to (poly)alkenes was developed earlier and validated for non-cyclic (poly)alkenes with respect to the total rate constants k(OH) and the detailed product distributions. Also for reactions of cyclic (poly)alkenes with OH, the SAR-predicted rate constant is in general within 15% of the experimental value, except for a few (bi)cyclic biogenic compounds; for both alpha-phellandrene and alpha-terpinene, k(lit) approximate to 1.7 x k(pred). For these two terpenes and for the structurally similar 1,3-cyclohexadiene we have measured the rate constants relative to the isoprene+OH reaction: k(alpha-terpinene+OH) = (3.4+/-0.4)x10(-10) cm(3)molec.(-1)s(-1): k(alpha-phellandrene+OH) = (3.3+/-0.4)x10(-10) cm(3)molec.(-1) s(-1): and k(1,3cyclohexadiene+OH) = (1.5+/-0.2)x10(-10) cm(3)molec.(-1)s(-1). The discrepancy between the SAR-predictions for OH-addition and the experimental k-values for the first two compounds -given their specific structure suggests an important H-abstraction channel in their reactions with OH. We determined the importance of the H-abstraction path by directly measuring the amount of H2O formed in the reaction with OH in Discharge-Flow/Molecular Beam Mass Spectrometry experiments. Thus, an H-abstraction contribution of 30 +/- 7 % was found for alpha-terpinene and 27 +/- 10 % for alpha-phellandrene, which explains most of the deviation of the total k(OH) with respect to the SAR-prediction for OH-addition.