This study was conducted to determine secondary school students’ conceptual difficulties in understanding of chemical equilibrium. The sample of study involved one hundred fifty-one 11th grade students from three different secondary schools in Izmir. For the data collection, chemical equilibrium misconception test (CEMT) and semi-structured interview were used. In the analysis of data, students’ responses in CEMT were categorized and also the data obtained from interviews were coded. At the end of study, it was revealed that students had some misconceptions about chemical equilibrium. Some of these misconceptions are parallel to the literature and some of them are originally found with this research. Some of the misconceptions, which we have not come across while reviewing literature are: When a solid substance is added to an equilibrium system at constant temperature, Keq increases, or decreases. When all reactants are consumed, the chemical reaction reaches equilibrium.