Stoichiometry and related concepts are an important part of student learning in chemistry. In this interpretive-based inquiry, we investigated Thai Grade 10 and 11 students' conceptual understanding and ability to solve numerical problems for stoichiometry-related concepts. Ninety-seven participants completed a purpose-designed survey instrument consisting of open-ended questions about stoichiometry-related concepts. Detailed analysis of the research findings suggest that most of the student participants held alternative conceptions similar to and different from those reported in the Western educational contexts. The alternative conceptions related to using a mole ratio as a mass ratio; identifying a limiting reagent as the least amount reactant; using 1:1 mole ratio for all substances in the equation; thinking that one mole of water was of 22.4 dm[superscript 3] in volume, and that the units mol/dm[superscript 3] represent the mole of solute per kilogram of solution. The students' ability to solve numerical problems seems to be related to their conceptual understanding of stoichiometry; students with alternative conceptions found it difficult to solve the problems relating to the concepts.