This research aims to review existing methods for assessing service sector policies and to propose a new framework for measuring liberalization and openness in services markets within regional trade agreements (RTAs). The research is grounded in theories of service policy and econometric modeling. It employs general scientific methods, a systemic approach, and economic-statistical techniques. Currently, there are two primary approaches to measuring the rate of services liberalization under RTAs: models based on the Services Trade Restrictiveness Index (STRI) and the Hoekman Index, which respectively compare de facto and de jure regulations. The author introduces an enhanced methodology, the Index of Services Market Openness (ISMO), designed to assess the openness of services markets under RTAs by incorporating a broad range of relevant factors. Although the ISMO model allows for a comprehensive analysis of RTA commitments, only the STRI-based approach currently captures de facto regulations. This highlights the need for further development of integrated methodologies to address the strengths and limitations of existing models. The research presents ISMO as a novel and comprehensive tool for evaluating services market openness under RTAs and the General Agreement on Trade in Services (GATS), and applies it to the analysis of the Belarus-China RTA on services and investment (2024).