This study aims to investigate the relationship between written and oral reasoning in an undergraduate chemistry laboratory course as part of an argument-based inquiry approach, which is also a generative learning environment, known as the Science Writing Heuristic (SWH). The study employed the data-transformation variant of convergent design of mixed-method research. Data sources included 180 laboratory reports from nine Pre-service Science Teachers (PSTs) and 20 video recordings across two semesters. Using Walton's argument schemes, PSTs' development, utilisation and correlation of written and oral reasoning and an argument cycle of premise-justification-conclusion were examined. A Friedman test and a Spearman-Brown correlation were conducted for statistical analysis. The results revealed that there is a positive correlation between written and oral reasoning. While the quality of PSTs' written reasoning significantly increased from the first time phase to the following time phases, this pattern was not observed in oral reasoning. An argument cycle of premise-justification-conclusion occurred in each phase of oral arguments. However, this cycle did not occur in every facet of the SWH process across all time phases in written arguments. The study suggests that pre-service science teachers should be provided with learning environments that will allow them to make external evaluations and engage in talking, reading and writing activities for learning purposes.