Deemed too politically and economically backward for membership during the EU's first eastward expansion in 2004, the Black Sea neighbours squeezed through the door in what political analysts say was the last chance to join this decade.
The accession of the poor, ex-communist duo raises the EU's membership to 27 states, almost half of them former eastern bloc countries cut off from the West by the Iron Curtain until 1989.
''Today a dream came true, a dream of generations of Bulgarians who have always wanted to live together with the free European peoples in peace and prosperity,'' Bulgarian Prime Minister Sergei Stanishev said at an outdoor concert in Sofia.
The EU's new borders will stretch from the Atlantic and Baltic in the west and north to the Black Sea in the southeast.
Romania -- the larger of the two -- and Bulgaria will together boost the EU's population by 30 million, to 490 million, but will add just 1 percent to its economic output.
Their membership has sparked debate over the EU's eventual borders, with some member states fearing further expansion could bring waves of immigration and crime that could drive their citizens out of jobs and lead to instability.
Other EU hopefuls such as Turkey and countries in the western Balkans now face the prospect of a long wait.
In France, where ''enlargement fatigue'' is particularly strong, President Jacques Chirac hailed the new wave of accession as a step in the reconciliation of Europe.
''Sofia and Bucharest are once again European capitals,'' he said, according to the text of a speech released by his office.
LONG AND DIFFICULT ROAD ''We are Home!'' said a headline in the Bulgarian newspaper Trud in a special New Year's day edition.
''You travelled a long and difficult road to get here. Welcome to the European Union family,'' German Foreign Minister Frank-Walter Steinmeier said after meeting Romanian officials.
Germany takes over the EU presidency on Jan. 1 and will face the monumental task of spearheading the bloc's institutional reform which is crucial to further enlargement.
Eager to tap 40 billion euros in EU development funds to overhaul dilapidated infrastructure and boost industry, Bulgaria and Romania hope membership will help them close a huge wealth gap with the West.
However, problems remain. Brussels has already criticised the new members for doing too little to combat corruption and -- particularly in Bulgaria -- organised crime gangs born from the ex-communist secret services that control large parts of the economy.
Although their economies are growing fast, income per capita is just a third of the EU average.
Diplomats fear that, having achieved EU admission, politicians may relax on reforms and Brussels has vowed to monitor the new members, threatening to penalise them if they fall behind.
As loud music from Sunday's celebration concerts echoed through the streets, Bulgarians and Romanians hoped their new status would mean an end to prejudice and isolation they felt as non-EU members.
''Until today when I have gone to other countries, every waitress, every salesman turned up his nose when hearing I come from Romania,'' said salesman Sergiu Radu, 27.
''I hope this means an end to that shame and frustration.'' REUTERS MS PM1538