De Halleux was actively involved in oecumenical dialogue, was appointed a consultor to the Pontifical Secretariat for Christian Unity in 1974, and took part in the Vienna 'Pro Oriente' colloquia.
In 1964 he founded the organization Pro Oriente, to promote the relationship between the Catholic and Orthodox churches.
There were several important exchanges of visits between Popes and heads of the Oriental Orthodox Churches, and semi-official theological conversations sponsored by the Pro Oriente Foundation in Vienna.
In 1970 he was appointed as honorary member of the foundation Pro Oriente.
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