Each company will invest €150m (£98m) to increase the capital of Sony Ericsson Mobile Communications, which reported losses up to half and revenues slightly up in the last three months of 2002.
During the final quarter of last year, the company shipped 7.1 million mobile handsets, a 4% increase on the same period a year earlier and a 42% increase on the third quarter.
"Last year we were introducing products but we didn't have a complete product line," said Ericsson spokeswoman Pia Gideon.
"Sales were mainly driven by the T68, which was very successful in Sweden and all over Europe, but it's not enough to have just one product. We are gradually launching more and more products and in the end of the fourth quarter launched the P800, which is our camera-phone."
During the fourth quarter, net sales totalled €1.2bn (£785m). Pretax losses ran to €77m and net losses were €69m, both of which are around half of what they were a year earlier.
Sony Ericsson is expecting to become profitable in 2003 and also expects to expand its market share, Gideon said. That will mean achieving unit sales growth of more than 10%, which is the company's estimate for the total market in 2003. It said it expects the entire market to increase from 395 million units sold in 2002 to 435 million this year.