<p>Urbanization is rapidly transforming ecosystem processes by altering environmental conditions, including increasing temperatures due to the Urban Heat Island (UHI) effect. Changes in environmental conditions, such as temperature, may alter the rates of critical ecosystem processes and functions, including the decomposition of leaf litter. Additionally, variability in leaf litter quality may interact with changing environmental conditions, further altering rates of leaf litter decay across urban landscapes. Thus, we used a gradient of heat vulnerability (a proxy for the UHI) across the city of Philadelphia, PA, USA to evaluate how urbanization and leaf litter quality may affect leaf litter decomposition rates. Specifically, at four sites ranging in their heat vulnerability index, we deployed tea bags (i.e., green and rooibos) and natural leaf litter bags containing either <i>Gingko biloba</i> (hereafter, gingko), <i>Liquidambar styraciflua</i> (hereafter, sweetgum), <i>Platanus occidentalis</i> (hereafter, sycamore) or <i>Quercus rubra</i> (hereafter, red oak). Heat vulnerability and leaf litter species both affected the rate of leaf litter decomposition. Specifically, as site heat vulnerability increases natural leaf litter decomposition decreases; however, tea bag decomposition was unaffected by site heat vulnerability. Additionally, natural leaf litter species decompose at different rates, with species having more labile compounds (i.e., gingko and sweetgum) decomposing faster than species having more recalcitrant compounds (i.e., sycamore and red oak). Overall, our findings contribute to the growing discussion regarding the effects of urbanization on ecosystem processes and highlight the complexity of predicting decomposition processes in altered landscapes.</p>