A study was made to isolate factors which have systematic and repetitive effects on the unionization of librarians, particularly the professional librarian in the public library. The historical patterns of library unionism are summarized, and an analysis is made of the personal characteristics of librarians, their economic position, and employment environment as these affect the propensity to unionize. The following conclusions are reached: 1) professional and employee associations have not played a major role in hindering unionization; 2) occupational characteristics were not found to be significant in explaining propensity toward unionization; 3) economic position of librarians has had a differential effect on the pattern of unionism; and 4) environmental factors found to be important in unionization were employment concentrations, legislative climate, and strong labor traditions. Appendixes contain the questionnaire distributed to professionals, factors of employee satisfaction, and statistical tests used in the study. A bibliography is included. (Author/KKC)