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Nearest neighbors queries are commonly used in location based services. This paper focuses on enhancing their processing based on indexing structures. Indexes, like R-trees, cannot avoid backtracking to process them. This feature weakens suitability of such an index to several data access modes within mobile environments. To meet needs of these environments, we propose a novel index called VN-tree (Voronoi-Neighbors tree), based on the Voronoi diagram and its dual graph (the Delaunay Triangulation) of the candidate points of interest. VN-tree adopts a non-overlapping partitioning scheme and supports backtracking-free search algorithms to answer several types of queries. In particular, processing continuous nearest neighbor queries is a straightforward use of VN-tree. We conduct various experiments to compare VN-tree with R*-tree as in-memory indexes in the on-demand data access mode. Main results show that VN-tree consistently provides outstanding performances.