This study aimed to develop a two-tier test to identify misconceptions of pre-service teachers about chemical equilibrium. The sample was made up of 135 pre-service chemistry teachers at Farhangian University in Iran (70 female and 65 male) who were spending the final semester of the eighth semester of the teacher training programme. After analysing the distribution pattern of the participants' answers in the first and second tiers, fifteen misconceptions were identified. A new misconception was identified for the first time, which we called the common ion effect which was held by about 50% of participants. Gender was a significant factor in the rate of misconceptions, with male preservice teachers having fewer rate misconceptions compared with females. The results showed that when the first tier or the second tier was considered alone, female participants performed better, but when both tiers were combined, the performance of males was better. However, males had a weaker performance in three questions related to the approach to equilibrium in this situation. These findings will help educators plan their instruction by knowing pre-service teachers' preconceptions about chemical equilibrium.