This study focused on students' misconceptions related to ozone depletion and global warming. Six open-ended diagnostic questions were administered to 103 randomly selected lower secondary school science students in Trinidad and Tobago. A subsample of 15 students was purposively selected and interviewed to clarify their written responses and to further probe their understandings based on their responses to the six questions. The findings revealed that students have a number of misconceptions about ozone depletion and global warming. The results have implicaitons for the teaching of these concepts at the lower secondary science level and for a review of the teaching resources and teaching strategies used.