This paper explores the potential integration of freight transport into a Mobility as a Service (MaaS) environment. MaaS is a user-oriented service concept providing door-to-door mobility solutions for people. It integrates services of multiple providers, enabling searching, booking, and payment through a single digital platform. Although MaaS is often presented as a great opportunity to increase the efficiency of the overall transport system, its application is constrained to passenger transport. The possibility of combining freight and passenger trips in MaaS might contribute to improving capacity utilization in passenger transport and to reducing freight movements in cities. The aim of the paper is to systematically explore the service models through which integration might take place and to qualitatively assess their potential impact. Based on a suitable conceptualization of what we called MaaS4PaF, i.e. MaaS for Passenger and Freight, we identify relevant logistics segments, propose service models and evaluate these from a multi-stakeholder and sustainability perspective. The analysis results in propositions about the fitness between on-demand freight and passenger systems combined in MaaS, as well as a related research agenda to further investigate the potential of this approach for future sustainable mobility.