The study was carried out to determine the effects of process skills instruction on formal reasoning ability of the subjects under study. The target population was SS II students drawn from selected schools in Kaduna state. To determine the effect of process skills on formal reasoning ability, 76 subjects were selected from two secondary schools in Zaria metropolis. The two schools randomly selected, were found after matching them, to be equivalent in their formal reasoning ability and level of acquisition of science process skills. One of the schools served as the control and the other served as experimental group was exposed to science process skills instruction while the control group which was exposed to lecture-based instruction. The two groups were both pre and post tested using the following tests: a. Test of formal reasoning ability b. Science process skills acquisition test The data were collected and analyzed using t- test statistics at 0.05 level of significant. Six hypotheses were tested ad the following findings were obtained: i. A high proportion of the subjects (74%) were operating at the concrete operational stage, ii. The experimental group performed significantly better than the control group in formal reasoning ability tasks after treatment, iii. Female subjects benefited from science process-based instruction more than their male counterparts, iv. The initial gap between the male and female subjects in formal reasoning ability was bridged in all the Schemata (conservation, control of variables, proportional reasoning, probabilistic reasoning, combinational reasoning) except in correlational reasoning, v. Science process skills instruction was found to be more effective in promoting the acquisition of science process skills in females than in their male counterparts, vi. Though the subjects showed an appreciable increase in their mean scores in the posttest over the pretest, the difference in the mean scores was, however, only significant in the schemas of combinatorial reasoning, proportional reasoning and control of variables, vii. Though both the concrete and formal operational subjects benefitted from the treatment, the concrete operational subjects seemed to have benefited more than their formal operational counterparts. Based on these results, one of the major recommendations made was that process-based instruction should be introduced in the early years in the secondary schools as a means to reduce, if not totally eliminate, the gender-related differences in reasoning ability.