The growing importance of argumentation in science education must also be accompanied by examples showing both teachers in service and trainee-teachers how to implement and assess argumentation in class. In this line, this study is framed within a broader research study on argumentation competency , which involves the participation of Preservice Elementary Science Teachers (PEST)s from 3rd year of the Primary Education Teaching Degree from the University of Malaga (Malaga, Spain). Specifically, this paper shows an argumentation task that involves the participation of 98 Spanish Preservice Elementary Science Teachers (PESTs), through production and peer assessment, in order for them to internalise the criteria of a good argument, thus improving their argumentation skills. The set task is drawn from a PISA 2006 test, which addresses the possibility of reducing the hardness of a lipstick by changing its composition. PESTs are required to conduct peer assessment, which will then be compared to teacher assessment, in order to analyse the ability of the former to identify and evaluate the elements of an
argument. Results show different levels of capacity for analysis and evaluation of argumentation by PESTs. In particular, they struggle the most to identify and evaluate evidence and justification. Likewise, the peer assessment-teacher assessment comparison reveals an overestimation by PESTs in relation to evidence and justification.