The characteristics of participatory institutions can be articulated in three main dimensions: input, process and output. The common assumption is that a dependency relationship exists, with process serving as a mediator between input and output. This paper puts the model to a rare empirical test drawing on a unique dataset of 70 Spanish advisory councils. Through a combination of exploratory factor and path analyses, we analyse the dimensionality of input, process and output and investigate the direct and indirect impact of inputs on process and outputs. Our analysis provides evidence that input factors have a direct impact on the output factor transparency, but their impact on effects on policy and participant satisfaction is mediated by the process factor deliberation. Further, the capacity of the public administration to steer the advisory council (wardship) mediates negatively the impact of input variables on transparency. The analysis provides a nuanced account of how different input and process design characteristics of participatory institutions have profound direct and indirect effects on their outputs.