Twelve Spanish-speaking adults learning English as a second language were tested using the Bilingual Syntax Measure and their errors were analysed. Eight of them were subsequently presented with their errors in written form and asked to correct them. The 12 adults produced a total of 451 errors, of which 20.6% were due to interference from Spanish, 60.3% were developmental and 19% were from other sources. There was no variation in the proportions of errors from different sources according to academic status: intermediate students produced 60.3% developmental errors and advanced students produced 60.2%. Most subjects, both intermediate and advanced, were only able to correct about half their errors. Their corrections of the "Is X" structure suggest that this error may be due to syntactic, rather than phonological, interference. (Author)