Na-ion batteries are considered interesting alternative to the Li-ion batteries due to the richer resources and lower costs of sodium as compared to lithium. The development at large scale of efficient electrode materials with satisfactory electrochemical properties is a determining factor. Iron and phosphate-based materials, such as the olivine NaFePO4, are good candidates for their use as positive electrode for Na-ion batteries since they deliver 120 mAh/g at a potential of ~3V vs. Na/Na+. However, materials working at higher voltage are highly desirable. In this sense, mixed Na[Fe,Mn]PO4 with olivine structure appear to be very promising. In this work, we would like to investigate the possible long-range arrangement of Mn and Fe atoms in olivine-type NaxFe1-yMnyPO4 electrode materials using neutron powder diffraction, and correlate it with the peculiar features observed in their electrochemistry.