A customs union linking Kenya, Tanzania and Uganda comes into force on Saturday following six years of negotiations.
Kenya has a more developed industrial base than its neighbours
The new trading bloc is a first step towards a common market and will gradually enable goods to pass between the three countries tax-free.
The member states hope it will also lead to a common currency and eventually a political federation.
The union is an attempt to revive the East African community which collapsed 10 years after it was set up in 1967.
Correspondents say the three countries' farm-based economies are considered stable when
compared to their war-devastated neighbours.
The treaty setting up the East Africa Community (EAC) Customs Union was signed last March.
Under its terms, Kenya will pay duty on its goods entering Tanzania and Uganda for the first five years, to compensate for its more developed industrial base.
Tariff levels against goods from countries outside the grouping will be harmonised.
BBC East Africa correspondent Adam Mynott says it has not been easy to get this far and on a number of occasions it appeared agreement would be impossible.
But the leaders of the three countries recognise that tariff-free trade promises increased economic growth, our correspondent says.
Ultimate goals also include an East African central bank.