There is sample evidence that work conditions affect employees' well-being. Losses in work quality (increased job stressors and reduced job resources) are thought to be related to deteriorations in well-being, whereas gains in work quality (reduced job stressors and increased job resources) are believed to improve well-being. The way most previous studies tested linkages between work conditions and well-being assumes that as much as a loss in work quality harms well-being, a gain in work quality results in an improvement. However, Hobfoll's conservation of resources (COR) theory argues that losses have a stronger impact than gains do. To date, this assumption still awaits a thorough empirical test. Using data from three longitudinal studies (Ns = 10,756, 579, and 2,441), we investigated the effects of changes in work conditions on well-being. Changes in work conditions were related to changes in well-being, and these relationships were weaker with longer time lags. Moreover, in line with COR theory, our analyses suggested that the effect of a loss in work quality was generally stronger than the effect of a gain. Interestingly, however, we found a more consistent pattern for the effect of certain stressors (e.g., social stressors) than others (e.g., workload). By testing a central principle of COR theory, this research advances theoretical understanding of how work affects well-being. Furthermore, by revealing that previous studies may have underestimated the detrimental effects of deteriorating work conditions and overestimated the positive effects of improved work conditions on well-being, this research also has implications for organizational interventions. (PsycInfo Database Record (c) 2023 APA, all rights reserved).