He specialised in the study of the abolition of the slave trade, and some have argued that his work was ground-breaking for the direction which it took.
In 1944 his book Capitalism and Slavery argued that the British abolition of their Atlantic slave trade in 1807 was motivated primarily by economics; by extension, so was the emancipation of the slaves and the fight against the trading in slaves by other nations.
Anterior to Williams the historiography of this issue had been dominated by (mainly) British writers who generally were prone to depict Britain's actions as unimpeachable.
The difficulty with Williams' argument is they are underdetermined by the facts of the matter. The decline in the West Indian economies began to manifest itself after slave trading was banned in 1807; prior to this, slavery was flourishing. The economic decline in the West Indies was consequently more likely to have been a direct result of the suppression of the slave trade. Williams' evidence showing falling commodity prices as a rationale can largely be discounted; the falls in price led to an increase in demand, raising overall profits for the importers.
Profits for the slave traders remained at around ten percent on investment and displayed no evidence of declining. Land prices in the West Indies, an important tool for analysing the economy of the area did not begin to decrease until after the slave trade was discontinued.
The sugar colonies were not in decline at all, in fact they were at their economic peak in 1807. It should be noted that Williams was heavily involved in the movements for independence of the Caribbean colonies and had a fairly obvious motive to impugn the colonial power.
A third generation of scholars led by the likes of Drescher and Anstey have discounted many of Williams' arguments. They do however acknowledge that morality had to be combined with the forces of politics and economic theory to bring about the end of the slave trade.