The relevance of the research is conditioned by the need to develop the theory and practice of integral assessment of the state and emergent properties of complicated natural and social systems, with the use of modern methods of evaluation. The article deals with assessment of stability of the social-ecological-economic system (SEES), being its integrative (complex, emergent) property influencing the "quality of life" of the population. The authors, analyzing a regional SEES, characterize it by the ability to retain its properties and mode parameters in case of external influence on the system or its intra-system changes affecting the "quality of life" of the population. Tver region of the Russian Federation was taken as a model object. To assess the stability, the authors devised the following scenarios of impact on each of the subsystems of a particular regional SEES: 1 - hypothetical aggravation of the ecological situation in the region by 10, 20, 30%; 2 - hypothetical aggravation of the economic situation in the region by 10, 20, 30%; 3 - hypothetical aggravation of social conditions in the region by 10, 20, 30%; 4 - hypothetical aggravation in all subsystems simultaneously by 10, 20, 30%. Further, in the scenarios 5 – 8, twofold aggravation of the situation takes place - alternately in all of the above subsystems and in all subsystems simultaneously. The integral indicators of quality of life of the population were calculated for all 8 scenarios - for the first (within the subsystems) and second (between the subsystems) levels of convolution of the indicators. The results of assessment of the environmental quality and the quality of social life are presented for the years 2003 and 2013. At the first stage, the linear character of changes, with equal weighting of the estimation parameters within the three subsystems (environmental, economic, social) and between them was taken into account. The ecological subsystem included 8 evaluation parameters; the economic subsystem - 5; the social subsystem - 5. These parameters were selected from the website of the Russian Federal State Statistics Service ("The Russian Regions" reports). The major objective of the research was identification of situations in which SEES, upon assigned impact on it, will not be able to maintain its properties and mode settings and will shift to another class, both in individual subsystems and within the system as a whole. The condition of the system and the "quality of life" of the population was assessed for 5 classes (I-high; II-above the average; III-average; IV-below the average; V-low), specific for it in 2013. All the changes in the scenarios were realized against the background of 2013.