Pere Mata i Fontanet (1811–1877) was the most important Spanish toxicologist in the nineteenth-century. However, he remained an invisible character outside Spanish borders. He was the author of the most influential Spanish treatise on legal medicine and toxicology, which had six editions but was never translated. His treatises did not include experimental results but rather a rhetorical discussion and a place where he discussed and claimed for changes to be made in those new sciences. His participation in famous trials such as poisoning cases contributed to increase his claimed authority as an expert. This paper will show that it was precisely during those trials, when experts had to face the puzzling questions of lawyers and jurors, that toxicology was built.