We report a study of the structural properties of Ni monolayers on W(110) using low-energy electron diffraction (LEED), spot profile analyzing LEED (SPA-LEED) and scanning tunneling microscopy (STM) at room temperature. Below a coverage of 0.4 monolayers (ML) we find a pseudomorphic (1x1) LEED pattern. Starting from 0.4 ML, LEED experiments show a (8x1) superstructure, followed by a (7x1) superstructure at a coverage of about 0.7 ML. These superstructures are interpreted as coincidence structures. A stripe pattern characteristic of the (7x1) structure is found with STM. STM indicates a partial transformation into the (7x1) coincidence structure:  10% of the first monolayer in the vicinity of surface defects remains pseudomorphic. Annealing the films leads to a lateral separation of the pseudomorphic and (7x1) coincidence structures, but not to the growth of one phase in preference to the other.