The scientific article is devoted to the psychological analysis of features of differentiation of concepts of untruth and lies in scientific researches of domestic and foreign experts-psychologists. Theoretical consideration of this problem led to appeal to the basic tenets of theories of lies available in modern psychological knowledge. The primary sources of analysis were the classics in this subject area of work K. Bond, V. Znakov, P. Ekman, V. Kazmirenko, K. Khalmetski, D. Sliwka, E. Feess, F. Kerzenmacher, W. Friesen, A. Mehrabian, O. Fry, T. Levin and others. The review allowed to identify the following psychological factors and sources that determine lies: biological, personal, situational. Lies and untruths are objective in nature – this suggests that they can reasonably be the subject of scientific (polygraphic) research and evaluation. The purpose of the article is to try to distinguish between the psychological interpretation of the essence of the concepts of «untruth» and «lie» within the interpretation of the results of polygraphic research. Methodology. During the preparation of the article, some general scientific and special scientific methods of cognition were used, in particular: historical, phenomenological, comparative, systemic, functional, generalization. Scientific novelty. The most distinctive features of a lie that need to be emphasized by a polygraph examiner are: the fact of distortion of the truth (a lie is an objective product of the subject’s speech activity on the falsification of information); the desire to mislead (the intent of a lie determines a certain goal and creates motivation to achieve it); awareness of the act of lying (the ability to correctly navigate the situation, determining their own behavior and continuously monitor current experiences); instrumental nature (lying acts as a means to an end, ie lying becomes for the subject one of the elements of the communicative arsenal, which is focused on the predicted result of actions); expediency (the purpose of using lies may be the desire to obtain material and psychological benefits (own, socio-reputational, etc.). Selected features should be summarized in three characteristic features of lies: the fact of distortion of truth; expediency. Conclusions. In reality, it is incredibly difficult for a polygraph specialist to feel the fine line between lies and untruths and to determine not only the objective peaks of polygrams, but also to determine their true motives, aspirations and intentions. At the same time, the solution of this difficult problem may be an appropriate response to the allegations of objectivity of instrumental methods of determining the guilt or innocence of a person in committing an offense. Keywords: polygraph; untruth; lie; deception; truth; deception.