English version of the 23-item ERIQ was translated and back-translated. Content validity was examined by five experts and face validity was examined by twelve key informants before being tested for construct validity with 828 workers from six garment factories. Predictive validity was assessed through the relationship between the ERI constructs and p s y c h o l o g i c a l h e a l t h o u t c o m e s i n c l u d i n g psychosomatic symptoms, state of anxiety, depression, and job satisfaction. The internal consistency of the Thai ERIQ was tested using the first survey (n=828), and test-retest stability was examined 2 to 4 wk later with a subsample (n=408). The results show that 2% of workers reported effort-reward imbalance (ERI ratio≥1). The Thai ERIQ has good content validity with a Content Validity Index of 0.95. Cronbach's alpha coefficients for the effort, reward, and overcommitment scales were 0.77, 0.81, and 0.66, respectively. The 2-4 wk stability of these three constructs was moderate (r=0.496-0.576, p<0.001). Overall, the factorial validity was demonstrated as the best model fit, with high values of the goodness-of-fit indices, using confirmatory factor analysis, indicating accordance with the theoretical constructs of the ERI model. Logistic regression analyses supported significant associations of reward with all psychological health outcomes (p<0.05). The findings suggest that the Thai ERIQ has adequate reliability and validity to investigate the psychosocial work environment. The Thai ERIQ can be applied to the Thai working population, particularly industrial manufacturing workers. (J Occup Health 2008; 50: 480-491)