These vermouths are closer to the original style of vermouth associated with Vermouth di Torino (although there are now also dry Vermouth di Torino).

The sweet vermouths are based on tend to be aged for two to three years, then fortified, sweetened, and infused with the producer's mixture of botanicals. The vermouth is then chilled to crystalize any tartrates, which are then removed during filtration. Caramel is added for colour. This process is identical to that used to make dry vermouth but with a base of heavier, sweeter wines, more sugar and more caramel.

European Economic Community, regulation 1601/91 stipulates two categories of sweet vermouth determined by their sugar levels and alcohol content:
Semi-sweet; between 90 and less than 130 grams of sugar per litre.
Sweet: 130 grams or more of sugar per litre (150 grams is common).

Different brands of sweet vermouth, and even different Vermouth di Torino, can vary tremendously in their flavour profile.