The crown prince has four sons by his second marriage, whom he disowned in 1997 when he severed all ties with their mother, Yuvadhida Polpraserth.
At the time the crown prince stated that they had renounced all their royal titles. But the palace continues to recognise their right to use the title HSH, or His Serene Highness, even though the boys, now grown up, are banished from Thailand and live in the United States.
That leaves some doubt over where they sit in line to the throne, even though most commentators believe they are no longer considered possible successors.
It is widely believed that the crown prince may have had another baby boy this year with his current mistress, who is likely to become his next wife.
Why does all this matter so much?
The world's longest-reigning monarch, King Bhumibol has been on the throne in Thailand since 1946
The issue is critical in a country where the monarchy is considered pivotal to political stability, and where King Bhumibol, who turns 87 this week, is in such frail health.
Crown Prince Vajiralongkorn appears to be sorting out his personal affairs before the succession, so that he can choose who will be his queen, and who will eventually succeed him.