The article tackles the relationship between labour market stratification and in-work poverty, through a comparative analysis of Argentina and Spain. Although it is usual to refer to its multiple determinants, the main hypothesis is that the labour market?s stratification plays a fundamental explanatory role on in-work poverty and that, even in different countries, a similar pattern, linked to labour market inequalities, can be identified. The paper draws on the construction of a typology of economic-occupational positions, performs a descriptive analysis of the labour market stratification and carry out binomial logistic regression models, using two different definitions of in-work poverty, both at the individual and the household?s level. The research relies on the Permanent Household Survey (Argentina) and the Survey of Living Conditions (Spain) microdata. The results show important differences in labour market?s stratification in Argentina and Spain. However, as the hypotheses states, the economic-occupational position is a fundamental determinant of in-work poverty in both countries ceteris paribus individual or households? characteristics? and that the same groups of workers (precarious and microenterprises? wage earners and lowskilled self-employed workers) concentrate the greatest odds ratios of poverty, whatever the definition adopted.