The purpose of this study was to examine the effectiveness of traditional versus guided inquiry (with problem-solving process and cognitive function training) on high school chemistry knowledge, science process skills, scientific attitudes, and problem-solving competency. Two classes of students were recruited from three classes of Grade 11 students at one school in North-eastern Thailand. Using a split-plot design, students were assigned to an experimental (N = 34) and a control group (N = 31), and were administered (a) learning achievement tests (chemistry knowledge, science process skills, and scientific attitude), (b) a problem-solving competency test, and c) tests of cognitive functioning. The findings showed that students' learning achievement and problem-solving competency in the guided inquiry group were significantly higher than in the traditional group. The effect of the new teaching method does not seem to stem solely from improvement in cognitive functioning. We attributed the improvement to greater flexibility in the amount of information provided by the teachers, more effortful processing by the students, and greater collaboration amongst the students.