The current study, undertaken in the Sultanate of Oman, explored twelfth grade students' common misconceptions in seven chemistry conceptual areas. The sample included 786 twelfth grade students in Oman while the instrument was a two-tier test called Chemistry Misconceptions Diagnostic Test (CMDT), consisting of 25 items with 12 items incorporating visual representations and eight items used sub-microscopic representations. In addition, nine items required participants to study the visual diagram(s) in order to answer the questions. The results confirmed several misconceptions that had been identified in previous studies, as higher percentages of the misconceptions were recorded compared to the findings of previous studies. Misconceptions receiving the highest percentages were associated with combustion, chemical equilibrium, and electrochemistry. The results also indicated that twelfth graders had a difficulty dealing with visual test items. These findings are serious alerts to the practice of chemistry education at the secondary level in Oman. Recommendations are given to improve the teaching of chemistry by taking research-diagnosed misconceptions into account when designing instructional materials and classroom activities. (Contains 8 tables and 4 figures.)