Research studies that deal with student ability to investigate and carry out inquiry oriented investigations often call for educational practitioners to pay particular attention to incorporating the skills of scientific inquiry in the process of teaching and learning. This has the aim of helping learners acquire the skills needed to become problem solvers and independent thinkers. One aspect of the inquiry practice that is directly related to student ability to carry out scientific investigations is the ability to handle and control experimental variables. This skill is commonly known as ‘control of variables ability’. Control of variables, as a process skill, has been widely regarded as an important ability in scientific investigations. The purpose of this study, therefore, was to assess how well students across educational grade levels develop this important process skill. Specifically, this study was designed to assess the understanding of the control of variables of selected sample of science students from grades 8, 10, and 12 and to compare these students in relation to the development of this ability across grade levels. Using an assessment framework developed and used by the American Association for the Advancement of Science (AAAS), 128 science students were tested to assess their understanding of ideas pertaining to control of variables as a fundamental integrated process skill. The findings revealed that students across grade levels exhibited alternative conceptions of key ideas related to control of variables as a fundamental ability such as testing hypotheses, selecting the appropriate experimental setup, handling more than two variables, and providing valid explanations to the expected outcomes of an experimental setup. These findings suggested that much work is needed to improve student ability to handle and control experimental variables particularly in the context of UAE Curriculum and recent educational reforms that stress the need for better preparation of students to meet the challenges of today’s changing societies. The findings have also highlighted that such a need for better preparation of students for the future scientific inquiries requires new curricula and teaching approaches that respond to and focus on not only learning essential scientific content but also on acquiring advanced transferable abilities related to scientific inquiry and logical reasoning skills that can be used to solve societal problems.