He set sail on May 9, 1540 with orders from the Spanish court to await at a certain point on the coast the arrival of an expedition by land under the command of Francisco Vasquéz de Coronado. The junction was not effected, though Alarcón reached the appointed place and left letters, which were afterwards found by Melchior Diaz, another explorer.
Alarcón was the first to determine with certainty that Baja California was a peninsula and not an island, as had been supposed. He made a careful survey of the coast, ascended the Rio del Tizon or Rio de Buena Guia (Colorado River) for 85 Spanish miles, and was thus able on his return to New Spain in 1541 to construct an excellent map of California.