In Papal oath: Information From Answers.com you can read the papal oath, put in place supposedly by St. Agatho, in AD 681 which was taken by new popes upon their coronation:
I vow to change nothing of the received Tradition, and nothing thereof I have found before me guarded by my God-pleasing predecessors, to encroach upon, to alter, or to permit any innovation therein;
To the contrary: with glowing affection as her truly faithful student and successor, to safeguard reverently the passed-on good, with my whole strength and utmost effort;
To cleanse all that is in contradiction to the canonical order, should such appear;
To guard the Holy Canons and Decrees of our Popes as if they were the Divine ordinances of Heaven, because I am conscious of Thee, whose place I take through the Grace of God, whose Vicarship I possess with Thy support, being subject to the severest accounting before Thy Divine Tribunal over all that I shall confess;
I swear to God Almighty and the Savior Jesus Christ that I will keep whatever has been revealed through Christ and His Successors and whatever the first councils and my predecessors have defined and declared.
I will keep without sacrifice to itself the discipline and the rite of the Church. I will put outside the Church whoever dares to go against this oath, may it be somebody else or I.
If I should undertake to act in anything of contrary sense, or should permit that it will be executed, Thou willst not be merciful to me on the dreadful Day of Divine Justice.
Accordingly, without exclusion, We subject to severest excommunication anyone — be it ourselves or be it another — who would dare to undertake anything new in contradiction to this constituted evangelic Tradition and the purity of the Orthodox Faith and the Christian Religion, or would seek to change anything by his opposing efforts, or would agree with those who undertake such a blasphemous venture.
This oath was taken by all the popes since 681 and all modern popes, including Pius X’s succesors, Benedict XV, Pius XI, Pius XII, John XXIII, and Paul VI. Yes, even the darling of Catholic liberals, John XXIII (again this convinces me that today’s liberals have a profound misunderstanding of John XXIII, who convened Vatican Council II), and his successor, Paul VI. The first pope to abandon the oath -he refused to take it- was John Paul I, who also refused to be crowned. Interestingly, John Paul II also refused to take this oath, and refused to be crowned.
What’s crucial here is if, on Sunday, Pope Benedict XVI does one of the following:
1. Have himself crowned with the papal tiara, or crown;
2. Take, before God and the public, the traditional papal oath.
Either act would definitively establish what kind of a papacy we would have: a new Counter Reformation. If he declines both, however, he may just turn out far more moderate and progressive than people expect.
The papal oath, however, shouldn’t be confused with another oath, also against modernism, required of the clergy by St. Pius X. That oath is here: The Anti-Modernist Oath. An adequate discussion of anti modernism is at Anti-Modernist oath – Learn all about Anti-Modernist oath.