Abstract:As global biodiversity trends worsen, protected area (PA) environmental effectiveness needs to be assessed to identify strengths and areas to improve. Through a participatory process including PA managers and scientists, we refined the System for the Integrated Assessment of Protected Areas (SIAPA), in order to increase its legitimacy, credibility and salience to end users in Spain. Then, we tested the optimised version of the SIAPA on two emblematic Spanish national parks (NPs): Ordesa y Monte Perdido NP (Ordesa NP) and Sierra de Guadarrama NP (Guadarrama NP). PA managers and scientists largely coincided in the ratings of SIAPA's indicators and indices. Collaboration with Ordesa NP's managers was regular, allowing a nearly complete evaluation of the NP. However, greater collaboration between PA managers and scientists remains a priority in Guadarrama NP. Results show that potential effectiveness is moderate for Ordesa NP and low for Guadarrama NP, according to the indicators that could be evaluated. For Ordesa NP, lack of data on focal habitats and other focal features determined a deficient valuation of its conservation state, although the remaining indicators in that category showed adequate or moderate values. The compilation of those data should be overriding in the NP. In contrast, only climate change posed a serious threat in that NP. The social perception and valuation of both NPs was good, suggesting broad support from local populations and eased management.