The painting has been in the private collection of a family for several years, but the museum would not release any more information about the owners because of privacy concerns, Mr. Rüger said. Two years ago, they brought it to the Van Gogh Museum to seek authentication, and researchers from the museum have been examining it ever since, said Mr. Rüger. The museum recently concluded that the work was a van Gogh because the painting’s pigments correspond with those of van Gogh’s palette from Arles.

It was also painted on the same type of canvas, with the same type of underpainting he used for at least one other painting, “The Rocks” (owned by the Museum of Fine Arts in Houston) of the same area at the same time. The work was also listed as part of Theo van Gogh’s collection in 1890, and was sold in 1901.

“Sunset at Montmajour” is comparable in size to van Gogh’s “Sunflower” painting of the same year. The owners brought it to the museum once before in 1991, said Mr. Rüger, but at the time no one recognized it as a van Gogh. “This time, we have topographical information plus a number of other factors that have helped us to establish authenticity. Research is so much more advanced now, so we could come to a very different conclusion.”