This paper examines the process of high school students' planning investigations in the chemistry laboratory across two consecutive academic years in terms of their actions and their progress. The context is a set of five inquiry-based laboratory tasks in which participants (9th and 10th graders, 14-15 and 15-16 years of age) are required to plan and carry out investigations to solve contextualized problems. Data collection includes audio and video recordings of students' conversations and actions, as well as students' written products. For the analysis, two different rubrics have been developed, based on students' discourse and actions. Rubric 1, examining the students' actions related to planning investigations, consists of five categories that correspond to some of the operations assessed in the competency of assessing and designing scientific inquiry (OECD, "PISA 2015 Draft Science Framework," 2013). Rubric 2, analyzing students' progress in planning investigations, consists of four levels of proficiency depending on: (a) the adequacy of their proposals to solve the issues addressed in each task and (b) their level of requirement of help from the teacher or not to complete the plan. The main findings point to a range of levels of achievement among groups and tasks, as well as in the type of scaffolding required from the teacher. Regarding the students' progression in the practice of planning, all groups evidence general progress from the beginning to the end of the study.