Science and Technology Parks (STPs) have proliferated in many countries as a policy initiative to foster innovation. Previous studies have mostly analysed average effects on the innovation performance of tenant firms, leading to mixed results. This paper aims to explore whether there are heterogeneous STP effects. Creating a novel database of Spanish patents generated on- and off- park, merged with firm and STP characteristics, we find that younger and larger firms benefit more from being on-park and younger and older STPs have a stronger effect on tenants’ patent quality, while STPs promoted by universities do not exhibit a differential effect.