The main objective of this research is to analyse consumers' purchasing behaviour in modern retail businesses in Bangkok Metropolis. The concept of duality in consumption is applied to back up this analysis with demography and socioeconomic variables, using the technique of multiple regression analysis. This study focuses on four modern retail businesses, namely Lotus, Watson, Top Supermarket and Big C. Consumer goods are classified into four types, which are shower gel, shampoo, body lotion and tissue paper. Results from the survey data show that the income elasticity is less than 1, which means that consumer goods are necessary in the view of people in Bangkok. Regarding the analysis of factors influencing the expenditure of consumer goods, the study reveals that the level of income is the most influential factor affecting the buying behavior for theproducts including shower gel, shampoo, and body lotion, whereas the number of family members is the most influential factor for the purchasing of tissue paper.