Objective: Exploring the feasibility to understand changes in oral hygiene behaviour using the Health Action Process Approach (HAPA) model applied to qualitative research interviews in patients with diabetes and periodontitis undergoing standard periodontitis treatment. Methods: Patients with type 1/2 diabetes and chronic periodontitis (n = 8) received standard non-surgical periodontal treatment accompanied with personalized oral hygiene instructions by a dental hygienist. Clinical indices (% bleeding on probing (BOP), probing depth (PD), clinical attachment level (CAL), % of sites with PD ≥ 5 mm, periodontal epithelial surface area (PESA) and periodontal inflammatory surface area (PISA) were recorded pre- and post-treatment. At 3 months post-treatment, patients were interviewed using a topic guide relating to oral health. A behaviour change framework was constructed from elements of the HAPA model and used directly to map interview data to evaluate oral hygiene behaviour in these patients. Results: Data from this feasibility study suggest a clinical improvement in periodontal status, albeit only monitored for 3 months. Application of the HAPA model highlighted the behavioural change pathway that diabetes patients undertake before, during and after periodontal treatment. The data suggest that patients move through all elements of the motivation phase and all elements of the volition phase except for the recovery self-efficacy element. Conclusion: The novel approach of applying the HAPA model to qualitative research data allowed for the collection of richer data compared to quantitative analysis only. Findings suggest that, in general, patients with periodontitis and diabetes successfully manage to incorporate new oral hygiene behaviours into their daily routine.