ABSTRACTThis study uses the relevant test results from the Second International Science Study and the Local G.C.E. ‘O’ level Chemistry papers to show some difficulties faced by Singapore students in understanding the mole concept. It also advocates that in teaching and learning the mole concept, teachers should make use of every opportunity to train students to “think in mole or number of particles” rather than to follow rigidly certain patterns to solve problems. “Think in mole” is the key to many problems, and hence the fundamental skill in solving all the problems in the mole concept.