Theo van Gogh (1857-1891) first worked as an art dealer in 1873 in the Brussels art gallery owned by his uncle Hendrick van Gogh. A few months later, he was sent to the Hague as an employee of Goupil & Co., an internationally-known Parisian merchant…

Vincent, his brother and elder by four years, preceded him in this career as early as 1869. He acted as Theo’s mentor…These exchanges continued in Paris, where Theo was sent for the 1878 World Fair. He settled there and around 1880 he became director of the Paris branch of Goupil & Co. As for Vincent, he left Goupil & Co. in 1876 and decided, after much hesitation, to become a painter. In 1886 he came to Paris, staying with his brother, before going two years later to the south of France.
FROM Theo van Gogh : art-dealer, collector, Vincent’s brother  ~Musée d’Orsay

The Letters From Vincent to Theo~
http://www.vggallery.com/letters/to_theo_main.htm

The Letters From Theo to Vincent~ http://www.vggallery.com/letters/to_vincent.htm