suMMary one way of classifying water quality is by means of indices, in which a series of parameters analyzed are joined a single value, facilitating the interpretation of extensive lists of variables or indicators, underlying the classification of water quality. the objective of this study was to develop a statistically based index to classify water according to the Irrigation Water Quality Index (IWQI), to evaluate the ionic composition of water for use in irrigation and classify it by its source. for this purpose, the database generated during the technology generation and adaptation (gat) program was used, in which, as of 1988, water samples were collected monthly from water sources in the states of Paraíba, rio grande do norte and Ceará. to evaluate water quality, the electrical conductivity (eC) of irrigation water was taken as a reference, with values corresponding to 0.7 ds m -1 . the chemical variables used in this study were: ph, eC, Ca, Mg, na, K, Cl, hCo 3 , Co 3 , and so 4 . the data of all characteristics evaluated were standardized and data normality was confirmed by lilliefors test. then the irrigation water quality index was determined by an equation that relates the standardized value of the variable with the number of characteristics evaluated. thus, the IWQI was classified based on indices, considering normal distribution. finally, these indices were subjected to regression analysis. the method proposed for the IWQI allowed a satisfactory classification of the irrigation water quality, being able to estimate it as a function of eC for the three water sources. Variation in the ionic composition was observed among the three sources and within a single source. although the water quality differed, it was good in most cases, with the classification IWQI II.Index terms: salinity, electrical conductivity, ionic composition.(1) received for publication in august 8, 2011 and approved in March 7, 2012. (2) agronomist, doctor in natural resources. Professor of the environmental and technological sciences department of the universidade federal rural do semiárido (rural federal university of the semiarid region) -ufersa. fellowship holder from the CnPq. e-mail: celsemy@ufersa.edu.br (3) agronomist, Master's degree in Irrigation and drainage, ufersa, Br 110, km 47, Costa e silva, CeP 59625-900 Mossoró (rn).