For abstract concepts to be well understood, students need to be familiar with the concepts and able to associate them correctly. Students cannot make scientific explanations on the subject unless they can establish relationships. This study aimed to determine the understanding level of 11th-grade high school students on "gases" in terms of chemical variables and any alternative frameworks on the subject of gases. A methodology, including both qualitative and quantitative design, was used for this investigation. Open-ended questions and group discussion methods were used for data collection. The questions were applied to 87 11th-grade students from three public schools. Group discussions were conducted with nine students. The students' understanding level of gases was statistically presented based on the predetermined categories. Besides, alternative frameworks on gases were described with direct excerpts taken from the students' statements. According to the results of the study, it was concluded that the students' understanding level of gases was low. In addition, it was observed that students were often inadequate in explaining gas-related events and that there were numerous unscientific errors in their explanations. Active learning methods should be designed to minimize the alternative conceptions of high school students on gases and focus on the conceptual and scientific understanding of students.