Sophie herself started to learn the Russian language and to study the Orthodox religion, which of course pleased the Empress. On June 28 Sophie was received into the Church in a great ceremony. The next day the bethrothal took place and Princess Sophia of Anhalt-Zerbst became Grand Duchess Ekaterina Alexeyevna. In Russia the father's first name is added to that of the child. The Empress had chosen her own mother's name, Catherine, and Alexis because it sounded Russian. Catherine was now the second highest ranking lady in the country.

Shortly after, Peter contracted the measles, then he started to show all the symptoms of small pox. The Empress herself nursed him during his illness, which left him pockmarked and with very little hair. He knew how unattractive he was, which eroded the little confidence he had. Catherine thought him to be a most pitiful creature and it was with dismay that she looked towards her wedding day. By now Peter drank excessively and his behavior became crude. The court was back in St. Petersburg, and after several postponements, the wedding took place on August 21, 1745 in the Cathedral of Kazan. The marriage however was not consumated, because of Peter's retarded physical development caused by his many illnesses. It was at this time that Catherine, who had never felt more isolated, wrote: "I should have loved my new husband, if only he had been willing or able to be in the least lovable. But in the first days of my marriage, I made some cruel reflections about him. I said to myself: If you love this man, you will be the most wretched creature on Earth. Watch your step, so far as affection for this gentleman is concerned, think of yourself, Madame."