Submicrometer sized pure cubic phase, Eu3+ doped, and Yb3+/Er3+ co-doped α-NaYF4 particles with octahedral morphology have been prepared in acetic acid. The acetate anion plays a critical role in the formation of such symmetric octahedral structures through its selective adsorption on the (111) faces of the products. The size of the as-prepared octahedra can be tuned by varying the amount of sodium acetate added to the acetic acid. A possible formation mechanism for these octahedra has been proposed. The doped α-NaYF4 octahedral submicrometer particles show down-conversion and up-conversion photoluminescence typical of these materials.