Expectancy-Value Theory predicts that expectancy of success and subjective task value (STV) underlie differences in motivation and achievement. This study investigated how gender and SES related to achievement mediated by expectancy of success, STV, and their interaction. The sample consisted of 396 participants in their final year of upper secondary education. Self-report measures were completed of expectancy, STV, gender and socio-economic indicators. These were linked to exit examination grades (A Levels). Only parental education was directly related to achievement however gender and SES were indirectly linked to student grades through expectancy, STV, and the expectancy-STV interaction. Males, students with a higher level of parental education, and students from households with a higher number of possessions, all performed better in their examination due to higher expectations; higher STV amplified these relations. Gender and SES differences in achievement can be partly explained by psychological factors, namely students' expectations of success and STV.