Here are the verses in question:
4 As Solomon grew old, his wives turned his heart after other gods, and his heart was not fully devoted to the Lord his God, as the heart of David his father had been. 5 He went after Ashtoreth the goddess of the Sidonians, and Molek the detestable god of the Ammonites. 6 So Solomon did evil in the eyes of the Lord; and went not fully after the Lord, as David his father had done.
7 On a hill east of Jerusalem, Solomon built a high place for Chemosh the detestable god of Moab, and for Molek the detestable god of the Ammonites. 8 He did the same for all his foreign wives, who burned incense and offered sacrifices to their gods.
9 The Lord became angry with Solomon because his heart had turned away from the Lord, the God of Israel, who had appeared to him twice. 10 Although he had forbidden Solomon to go after other gods, Solomon did not keep the Lord’s command. (1 Kings 11: 4 - 10)
A commentary I was reading says this:
...it is not stated that Solomon himself offered sacrifice to these idols...the words "went after Ashtoreth," etc., no more involve personal service than the word "built" in ver. 7 involves personal labour; but both expressions show that he regarded these idolatries not only without disfavour, but with positive approval and practical encouragement.
Another commentary says:
Went after - This expression is common in the Pentateuch, and always signifies actual idolatry (see Deuteronomy 11:28; Deuteronomy 13:2; Deuteronomy 28:14)
But the first commentary rebuttals by saying this:
He cites Deuteronomy 11:28; Deuteronomy 13:2; Deuteronomy 28:14; but it should be considered that in the two passages last cited the words are added, "and served them."
So I've come to the conclusion that Solomon himself did not personally offer sacrifices at these altars. His wives did. But it was still considered idolatry on Solomon's part because he was the one who allowed the high places to be built, and if his heart was fully after God, he wouldn't have allowed that.
Is this a reasonable conclusion for me to make?