This article covers the relationship between the National Labour Confederation of Spain and the Comintern and its union adjunct the Profintern, from the Confederation's initial support for the October Revolution to its subsequent outright rejection of communist politics, with reference to the positions adopted by revolutionary syndicalist movements in other countries. During this period a small number of individuals attempted to tie the Confederation to the Communist International, but failed. The article covers an important period in Spanish labour history, and helps to explain the mistrust that would bedevil the Spanish revolutionary working-class movement until the Civil War. Previous research has presented the battle for control of the CNT as a straightforward battle between anarchists and communists. This was not the case. The pro-communists were a miniscule faction, led by men recently affiliated to the CNT and who had no understanding of the depth of rejection of politics by Confederal militants. They only managed to take control of the national committee by chance. Aware of their weakness they were forced to act in a secretive and often underhand manner. Using material not consulted in previous studies this article shows the extent of their subterfuge and of the opposition this created in the Confederation, as well as demonstrating that the CNT was not the only revolutionary organisation to reject the Bolshevik International.