During the first day of his visit, Rehn will meet Croatia's top officials and take part in a conference focusing on the fight against corruption and the reform of the judiciary.
On Friday, the European Commissioner will tour the eastern cities of Osijek and Vukovar.
Commenting on Croatia's accession negotiations with the European Union, Degert told the press that the talks were progressing very well.
Asked by reporters about the Wall Street Journal newspaper's claims that Croatian Prime Minister Ivo Sanader manipulated the media by citing only the positive sides of the EU's progress report on Croatia of 8 November, Degert answered that it was normal that one saw and emphasised positive sides, but that the Commission unequivocally expressed shortcomings and problems in Zagreb's membership talks.
Degert also announced a ceremony of signing an agreement on sugar quotas for Tuesday in the Croatian Agriculture Ministry.
Under the agreement, Croatia will be entitled to export 180,000 tonnes of sugar annually to the EU market.
So far, Croatia has been able to export unlimited quotas to the EU without any tariffs in compliance with the Stabilisation and Association Agreement.
Commenting on a new treaty on the Central European Free Trade Agreement (CEFTA), which is to be ratified by mid-December, the European Commission's official welcomed the deal, expressing hope that until 19 December all countries in concern would ratify the document.
The treaty on the amendment of the CEFTA and on admission to this group was initialled on 9 November in Brussels by representatives of Albania, Bulgaria, Croatia, Macedonia, Moldova, Montenegro, Romania and the UN Mission in Kosovo.
Degert admitted that there were some problems with Bosnia-Herzegovina and Serbia, which have refused to initial the Brussels document insisting on being granted more favourable conditions.
Bosnia is trying to ensure solutions for some agricultural products that would be more favourable than the existing ones in its trade agreements with Serbia and Croatia.
Serbia has insisted on more favourable conditions for its tobacco industry.
The European Commission is not involved in the agreement because it is a treaty regulating relations between the countries in the region, and we support the agreement, Degert said.
It is necessary to hold negotiations so as to reach a compromise and we believe that all parties concerned will do so, he added.
Degert also announced the opening of an EC information centre for Croatians in Zagreb on 11 December.