The 1690 expedition led by Alonso de León into present day Texas proved to be a pivotal journey that had lasting effects on the development of Spain's land north of the Rio Grande. This expedition established the first Catholic mission in the area. Also, La Salle's abandoned settlement was burned, and several Frenchmen living among the Indians were captured and returned with the expedition party to Mexico. The bartering for the release of some of these Frenchmen resulted in a skirmish in which four native Indians were killed. In addition, De León chronicled a great amount of information about the land through which he traveled, leaving a lasting diary recording his experiences as well as offering a glimpse into the then unsettled lands in present day eastern Texas. The 1690 expedition diary exists in the form of six manuscripts, and their analysis is the focus of this thesis. No scholar has ever taken into consideration all six manuscripts when conducting research regarding this expedition, and therefore research conducted thus far is not thorough. A comparative analysis of these six manuscripts is undertaken in this thesis, and the manuscripts are classified as revised or unrevised. Foster (1997) was the first scholar to classify manuscripts of the 1690 expedition as unrevised and revised. He classified only the Beinecke manuscript as revised, but this thesis also incorporates two other revised manuscripts unknown to Foster, the Gilcrease 67.1 and Gilcrease 67.2. The unrevised manuscripts included in this study are the AGI, AGN, and BNMex manuscripts. Three semi-paleographic transcriptions of manuscripts of Alonso de León's 1690 expedition diary are also presented. The AGI and Beinecke manuscripts are transcribed and an in-depth comparative analysis of the unrevised and revised manuscripts is completed. This analysis presents the numerous discrepancies that exist between the two families of manuscripts. Also, a transcription of the Gilcrease 67.1 manuscript is included to present a document previously unknown to scholars. The findings of this thesis should be of interest to scholars in many different fields of study who have interest in this time period and this region of the U.S. Southwest.