This paper describes a series of psychological experiments used to test a new data base query language. The intent is to make psychological testing of a language part of the design and development process. By testing a language while it is still under development, features that require changing can be identified and the changes made. The experiments, which used cOllege students as subjects, investigated: 1) overall “learnability” of the language, 2) “learnability” of individual features of the language, and 3) the types and frequencies of errors made. Recommended changes to the particular language and the basis for those recommendations are described. In addition, errors made by experimental subjects during the testing are then analyzed from the point of view of possible causes. Based on this analysis, a preliminary model of query writing and two possible indices of query complexity are suggested. Copyright © 1977 by The Institute, of Electrical and Electronics Engineers, Inc.