Letters in the Earl of Kincardine's papers show that William Bruce was in exile in Rotterdam during the 1650s with his cousin, Alexander Bruce, brother of the Earl of Kincardine. As Episcopalians, William and Alexander would have sought refuge from the Puritan Commonwealth established by Oliver Cromwell. In Rotterdam, they were in contact with Sir Robert Moray, a soldier and natural philosopher close to Charles II, who then resided at Maastricht. William Bruce was a merchant, based in the Scottish community in Rotterdam, but traveling widely. He owned a ship with Alexander Bruce and John Hamilton of Grange, and was involved in the trade of wine, coal, and timber between Norway, France, England, Scotland and the Low Countries. He is recorded as having a house and a mistress in La Rochelle. He may have had a son Normand by this mistress since in 1672 he figures as a witness to the baptism at Holyrood of a William Bruce, son of Normand Bruce, mason. Moreover, the marriage record of Normand Bruce states that he was employed at Balcaskie. In 1658, William and Alexander traveled together from Bremen overland to Maastricht to meet Moray. Alexander Bruce and Moray were founder members of the Royal Society in 1660, and it is likely that architecture featured in their discussions, particularly the new town hall in Maastricht that Moray had recently advised on.