* Mitsui, Tokyo Gas buy 76 pct of electricity plants

* Call option for remaining 24 percent

* Deal completes Gas Natural's asset sale plan to cut debt

* To launch new strategic plan in H1 2010

(Adds detail, background)

MADRID, Dec 24  - Spain's Gas Natural SDG SA (GAS.MC) sold electricity generation assets in Mexico to Japan's Mitsui & Co Ltd (8031.T) and Tokyo Gas Co Ltd (9531.T) (9531.T) for $1.2 billion, completing an ambitious asset disposal plan to reduce debt.

Gas Natural said the deal would not generate significant capital gains but would give it a more balanced exposure to the Mexican market, where the gas and electricity group is a leading player with 1,570 MW of installed capacity and 500 MW in construction.

Mitsui and Tokyo Gas will buy 76 percent of a string of generation plants, financed through a capital hike to be made in early 2010, and will have a call option on the remaining 24 percent.

The sale brings to a close Gas Natural's 3.6 billion euro divestment plan designed to cut a 22 billion euro debt pile resulting from its acquisition of rival utility Union Fenosa.

Still pending is its sale of 2,000 megawatts of regulated combined cycle capacity in Spain.

Gas Natural shares were untraded as the Madrid market is closed. They ended on Tuesday up 1.1 percent at 14.93 euros, having risen as high as 14.945 euros, their highest since mid September. (Reporting by Tracy Rucinski and Carlos Ruano; Editing by Karen Foster and David Holmes)