The aim of the study was to examine and to compare the relationships of different personality traits, subject-specific motivational dimensions and students' achievement in math and mother tongue in general upper secondary education, as well as how these variables predict their achievement. A total of 397 students attending the first year of general upper secondary education in Slovenia participated in the study. Different measures were used to assess students' personality traits, subject-specific interest, self-efficacy, volitional strategies and final grades in math and mother tongue. The results of the research showed different patterns of achievement predictors in both subjects and differences in the predictive power of included variables according to subjects. The included variables predicted 18% of variance in the Slovene language and 31% of variance in math. In both subjects, achievement was significantly positively predicted by self-efficacy and interest, and significantly negatively predicted by energy and procrastination. In math, agreeableness was also found as a significant negative predictor, and in Slovene language conscientiousness and immediate action were found as positive predictors of students' achievement. Implications of the study's findings are also discussed.