By the end of their high school studies, students should be able to understand macroscopic and sub-microscopic conceptualization of acid-base behavior and the relationship between these conceptual models. The aim of this article is to ascertain whether grade-12 students have sufficient background knowledge to explain the properties of acids, bases, and salts from their constituent ions or molecules. In addition, this research seeks to discern if certain learning difficulties are due to this lack of knowledge. Two concept maps are used for evaluation including the conceptual and procedural knowledge of acid-base behavior of substances. The diagnostic instrument consists of five questionnaires and two interviews. This study reveals that grade-12 students lack some of the necessary knowledge and skills to understand the acid-base behavior of substances. This could explain why students cannot differentiate between an ionic and non-ionic substance and that they associate the existence of H or OH in the formula with acid or basic reaction, respectively, without differentiating between atoms and ions. Students do not understand the different types of particles or the meaning of the subscript or superscript in a polyatomic ion formula. They do not differentiate between the ion and atom of an element and consequently find it difficult to interpret the properties of substances such as solution in water and electrical conductivity. (Contains 2 figures and 4 lists.)