This article offers new empirical evidence regarding the limits of the premises of educational privatisation policies. Educational markets rely on the assumption that private participation has the potential to boost school diversification and open new educational opportunities for students from disadvantaged backgrounds. Using a fresh empirical approach, we draw on the Chilean case to investigate this rationale, which remains a controversial issue in the global educational discussion regarding privatisation. We examine the assumption of diversification by analysing three aspects: the attributes used by private subsidised schools to position themselves in the educational market ('Value Proposition'), the extent to which they embody these stated attributes and the alignment between them and parental preferences. Based on surveys and administrative records, our findings suggest that although schools display clear identities to position themselves in the educational market, there is also a mismatch between what some types of schools offer and what families receive. Our data also reveal a high level of social stratification between school types which cannot only be explained based on parental preferences or school admissions. We interpret the unfulfilled promise of diversification as a failure in the privatisation process. Based on these findings, we discuss the policy-related factors of incoherent and unequal diversification in the Chilean educational system.