Molecularly defined and classical heterogenous Mo-based metathesis catalysts are shown to display distinct and unexpected reactivity patterns for the metathesis of long-chain α-olefins at low temperatures (< 100 °C). Namely, catalysts based on supported Mo oxo species, whether prepared via wet impregnation or surface organometallic chemistry (SOMC), exhibit strong activity dependencies on the α-olefin chain length, with slower reaction rates for longer substrate chain lengths. In contrast, molecular and supported Mo alkylidenes are highly active and do not display such dramatic dependence on chain length. 2D solid-state NMR analyses of post-metathesis catalysts, complemented by molecular dynamics calculations, evidence that the activity decrease observed for supported Mo oxo catalysts relates to the strong adsorption of internal olefin metathesis products due to interactions with surface Si-OH groups. Overall, this study shows that in addition to the nature and the number of active sites, the metathesis rates and overall catalytic performance depend on product desorption, even in the liquid phase with non-polar substrates. This study further highlights the need to consider adsorption when designing catalysts and the unique activity of molecularly defined supported metathesis catalysts prepared via SOMC.