This study identified and quantified chemistry conceptions students use when solving chemical equilibrium problems requiring application of Le Chatelier's principle, and explored the feasibility of designing a paper and pencil test to accomplish these purposes. A 10-item pencil and paper, two-tier diagnostic instrument, the Test to Identify Student Conceptualizations (TISC), was developed and administered to 95 second-semester university general chemistry students after they received regular course instruction concerning equilibrium in homogeneous aqueous, heterogeneous aqueous, and homogeneous gaseous systems. To validate TISC, nine students from a stratified random sample were selected for interviews incorporating comparable chemistry problems. The probability that TISC correctly identified an answer given by a student in the interview was p ϭ .64, whereas the probability that TISC correctly identified a reason given by a student in the interview was p ϭ .49. Eleven prevalent incorrect student conceptions about chemical equilibrium were identified by TISC. Students consistently selected correct answers more frequently (53% of the time) than they provided correct reasons (33% of the time). The level of association between student answers and their respective reasons on each TISC item was quantified using conditional probabilities calculated from logistic regression coefficients. The Kuder-Richardson 20 reliability for TISC was .79.