The transaction makes it one of the largest shareholders in the business.
A stock market filing yesterday showed the London branch of UBS's investment bank now has more than 8.2 million shares in Aggreko.
That is equivalent to 3.05% of the issued share capital of the company and above the 2% threshold which typically triggers a market notification.
M&G Investment Management currently has the biggest stake in Aggreko with 5.9%, followed by Baillie Gifford with 5.02% then billionaire Alastair Salvesen who has 3.72%.
UBS's stake puts them in fourth place just ahead of Capital World Investors with 2.75% and Legal & General Investment Management which has 2.59%.
Shares in Aggreko, which is run by Rupert Soames, closed up 4p, or almost 0.3%, at 1569p.
At that price UBS would have had to pay in the region of £129 million to acquire all of its holding in the Dumbarton business.
Aggreko supports a number of major events around the world as well as providing temporary power in areas with poor infrastructure or which have been hit by disaster.
Its generators were recently used at the Superbowl in the US. The equipment will also be deployed at the football World Cup in Brazil this summer and the Commonwealth Games in Glasgow.