In 1867 he traveled to Europe, spending time in both London and Paris. He then returned to Washington and in 1868 published a travel book, "Europe in the Summer-Time". This was followed by U.S. Secretary of State William H. Seward commissioning Poston to deliver the Burlingame Treaty to the Emperor of China and to study irrigation and immigration in Asia. After China, Poston continued to India where he developed a fascination with the Parsi people and Zoroastrianism. He reached Egypt by early 1869 and was in Paris by April of that year. After living a year in Paris he moved to London where he spent the next six years. During this time he worked as an editor of a London newspaper, foreign correspondent to the "New York Tribune", and as a "counselor-at-law". Poston also wrote several books during this time, publishing "The Parsees" in 1872, "The Sun Worshippers of Asia" in 1877, and his poem "Apache Land" in 1878.