In recent science education, experimentation features ever more strongly as a method of inquiry in science classes rather than as a means to illustrate phenomena. Ideas and materials to teach inquiry abound. Yet, tools for assessing students' achievement in their processes of experimentation are lacking. The present study assumes a basal, non-exclusive process model of inquiry in experimentation that can be considered a consensus from multiple approaches: (1) finding an idea/hypothesis, (2) planning and conducting an experiment, and (3) drawing conclusions from evidence. The study confronted 339 secondary level students with three "guided inquiry" experimentation tasks on 3 days. Selected working groups were videotaped while experimenting. All the students reported their processes in a structured report form simultaneous to their progress. The generated videos and reports were analysed in a two-stepped way: (1) Experimentation was coded according to the process model into process plots; on basis of these, (2) process-focused performance scores were calculated considering logical coherence and immediacy of the inquiry processes. Correlative analyses show for two of the tasks that the report format yielded comparable performance scores to those generated from video data after students have had opportunity to learn the surveying formats (r[subscript S]?>?0.80). A first suggestion of a process-oriented assessment tool for inquiry in experimentation can be drawn from this study. It might be used to inform and complement secondary science instruction.