The purpose of this study was to determine Turkish primary teacher candidates' conceptual understandings of gases. In this study, a survey approach was used. The sample consisted of 240 primary teacher candidates. As the data collection tool, a questionnaire of six questions consisting of three open-ended and three multiple choice was used. The validity and reliability of the questionnaire were provided using the opinions of three field experts and the reliability analysis for multiple-choice ones. The responses to the open-ended questions and the drawings were analyzed descriptively using the predetermined codes and then the categories were formed. The findings showed that primary teacher candidates had some common misunderstandings of gases and their level of understanding of gases is rather low. It is suggested that more attention should be given to developing a correct understanding of the fundamental science concepts in primary teacher training.