Melissa D. Smith has started in her role as CEO of Wex Inc., which processes fuel payments for fleet vehicles.

The company previously announced in May that Smith would become CEO in January. She succeeds Michael E. Dubyak, who is taking on the newly created role of executive chairman. Dubyak will hold that position through year’s end.

Smith, who started at Wex in 1997, has also become a board member and will keep her position as president.

Shares of Wex, based in South Portland, Maine, finished 2013 at $99.03 per share. They rose almost 30 percent for the year.