Ukrainian President Viktor Yushchenko has met his Polish counterpart, Alexander Kwasniewski, at the start of a two-day visit to Warsaw.
Viktor Yushchenko, left, considers Mr Kwasniewski an important ally
Mr Kwasniewski was among the mediators who went to Ukraine last year to help resolve the political crisis caused by disputed presidential election.
Mr Yushchenko went on to win a re-run of the ballot.
As well as thanking Poland, Mr Yushchenko is expected to discuss his country's efforts to join the EU.
"I appreciate very much what Poland and, most of all the president, are doing in supporting our efforts to join the European Union," Mr Yushchenko told Poland's Gazeta Wyborcza newspaper ahead of his visit.
Poland is considered an important ally of Ukraine in its bid for membership of the European Union, says the BBC's Helen Fawkes in Kiev.
Later, Mr Yushchenko is due to meet Polish Prime Minister Marek Belka, and to attend a concert launching the Year of Ukraine in Poland
On Tuesday, he will travel to the south-eastern village of Mlyny to visit the grave of the Ukrainian poet who composed the country's national anthem.