This study was designed to analyze the procedural fluency and written-mathematical explanation to select constructed response tasks of students in Thermodynamics problems. The study made use of 2 sections, composed of 26 students, in University Physics 1 to conclude on the research problem. It made use of the assumption that mathematical and English abilities control the students' performance to problem solving. Using Pearson-r and One-way ANOVA, it was found out that their procedural fluency on constructed response tasks is significantly related to their written-mathematical explanation ability, and a significant difference on their performance when grouped according to their mathematical and English ability. Bonferroni Correction Post Hoc Test results confirmed the assumptions of the study: the students' procedural fluency is dependent to their mathematical ability, both algebraic and trigonometric, while their written-mathematical explanation is associated to their English ability.