This paper investigates cumulative effects of strengthening and lengthening on English vowels across two prominence-bearing prosodic factors, phrasal accent and prosodic phrase boundary. F1, F2 and duration measures are compared across vowels in three prosodic contexts: ip-medial unaccented, ip-medial accented, and ip-final accented. The results show that for most vowels there is only one degree of vowel strengthening, conditioned by phrasal accent, without any additive strengthening effect of prosodic phrase boundary. Lengthening is observed in both accent and added phrase boundary conditions, and the effect is consistently cumulative for at least some vowels, suggesting a gradient increase of duration as a function of the strength of prosodic structure. This finding also provides compelling evidence that strengthening and lengthening effects are two independent mechanisms that serve to mark prosodically strong positions.