The first purpose of this study was to examine the associations between students' motivational characteristics and their language-specific grit for learning English. Second, this study aimed to investigate how students' language-specific grit and motivational characteristics related to their achievement in English. While examining the presence of associations sought through both of the research questions, a particular overall purpose was to identify what specific variables included in the analyses had the strongest impact on group differentiation concerning the dependent variables of grit and achievement. The participants included 182 students enrolled in a tertiary English preparatory program in Turkey. Chi-squared automatic interaction detection (CHAID) algorithm as a data mining method was used to analyze the data. The results revealed that task value, self-efficacy and intrinsic motivation had significant impacts on differentiating the students with different levels of grit. Moreover, language-specific grit, test anxiety, self-efficacy and control beliefs significantly related to students' achievement.