Studies have shown that in the study of Chemistry, students perceived bonding, chemical reaction and particulate nature of matter to be difficult. This is reflected in poor performance in tests involving these concepts. It seems that the Federal Ministry of Education, recommended instructional strategy, guided discovery is not yielding the desired result. Considering the importance of Chemistry to man and his environment, there is the need to inquire into ways of ensuring that students make meaning of what they learn in chemistry. Science educators now advocate for instructional strategies like concept mapping that enable students construct meaning of what they learn. In this study the effectiveness of concept mapping strategy in bringing about understanding of chemistry concepts is compared with that of guided discovery. The effects of the study instructional strategies on some variables (gender, home factors, reasoning ability, reading comprehension and attitude) were investigated.The study sample consisted of 246 (SSI) chemistry students drawn from 3 randomly selected secondary schools in Niger State of Nigeria. The students were assigned into two groups - concept-mapping and guided discovery. The conceptmapping group was taught using concept mapping strategy and the guided discovery group taught using guided discovery strategy. Both groups were pre- and post tested. Eight hypotheses were tested. The major instruments for the study were Chemistry Attitude Questionnaire, Home Factor Questionnaire, Chemistry Concept Achievement Test, Reasoning Ability Test and Reading Comprehension Test. The data obtained were analysed and tested at a significant level of 0.05. Findings of this study are: i. that the concept mapping group performed significantly better and retained more of the selected chemistry concepts than the guided discovery group, ii. there was no significant difference in the performance of boys and girls taught using concept mapping and guided discovery strategies. However there was a significant interaction effect between gender and instructional strategy attributable to concept mapping strategy, iii. there was no significant relationship between any of the home factors and performance in chemistry. iv. that concept mapping strategy improved reasoning ability, reading comprehension and students attitude to chemistry,v. that reading comprehension and reasoning ability contributed to students performance when concept mapping strategy was used. Based on these findings, recommendations were made for teacher education and the curriculum.