Ruddlesden Popper phases where n = 2 or 3 e.g. La4Ni3O10 are receiving increasing interest as potetnial fuel cell cathodes. However the structural and oxygen non-stoichiometry of these phases at elevated temperature is not well understood in comparison to the La2NiO4 phase. Additionally the n=2 and 3 phases are likely to have oxygen vacancies and hence the in-situ structural phase evolution will differ from the interstitrial containing n=1 phase. In this work we aim to probe the defect structure of the n=2 and 3 phases over the temperature range of RT - 800C focussing on the evolution of the occupancy of the crystallographic oxygen positions, correlating these data with previous work on the Ni redox, conductivity and oxygen diffusion behaviour. A full Rietveld and MEM analysis of these oxides will be undertaken.