Purpose—This study aims to demonstrate the antecedent factors of consumers’ cosmetics purchasing behaviour in the emerging market of Thailand from the perspective of sustainability. Specifically, the study aims to quantitatively analyse the impact of three hypothesised antecedents of consumer behaviour: product quality, communicating sustainability, and ethical business behaviour. Methodology—A quantitative methodology is applied in the study, which collects survey data from Thailand. This study focuses on two cosmetic brands in Thailand, a domestic brand and an international brand. The total 800-sample dataset was analysed using Structural Equation Modelling to validate a conceptual model with measurements of three antecedent factors: quality, ethical behaviour, and communication sustainability. Findings—It is found that ‘ethical behaviour of the producers had a non-significant impact for all samples and the ‘domestic brand’, whereas communicating sustainability had a significant impact in all sample cases. The proposed measurement scales present a practical and pioneering tool for assessing consumer responses and behaviour towards cosmetic brands. The set of scales will also help cosmetics marketers to appraise their strategic planning and monitor their progress toward creating and identifying consumer loyalty to cosmetics brands via producers’ ethical behaviour and CSR messaging. Originality—The global market and inter-stakeholder communications have greatly changed the way people perceive, behave towards, and react to business suppliers. The understanding of consumer brand loyalty in the cosmetics industry and the business strategies focused on the impact of communicating sustainability with ethical behaviour remain limited, especially in the context of the emerging market. This research contributes to filling this gap with empirical analyses.