The wind farm, which is located in Carmarthenshire, is expected to go into construction early next year.
Mynydd Betws is ESB's third UK wind farm acquisition in two years. Last year it acquired the West Durham and Fullabrook, Devon wind farms, with a capacity of 24MW and 66MW respectively. Fullabrook is set to be constructed this year.
ESB head of wind development Andy Paine said the acquisition was part of the company's aim to increase its presence in the UK wind sector.
He added: "With Fullabrook and Mynydd y Betws providing 100MW of development assets, together with 24 megawatts operational at West Durham, ESB is well on the way to achieving 200MW of wind generation in Britain by 2012. We are committed to developing a balanced portfolio of generation in this market with less reliance on carbon.
"ESB is delivering on its strategic objectives to become a significant investor in the UK renewables market."
Speaking about the deal, Eco2 commercial director, Darren Williams said: "The sale allows Eco2 to invest funds into its other businesses including its development of a Welsh tidal business."