The article is devoted to the problems of proving the reality of an economic transaction in tax legal relations. The concept of the reality of an economic transaction is absent in the Tax Code of Ukraine. However, in practice, this concept is widely used by controlling authorities during tax audits. In the way of recognizing the economic transaction as unrealistic, the controlling authorities create the prerequisites for adding to the taxpayer the amounts of liabilities that, from the point of view of the controlling authorities, were understated by the taxpayer. A significant part of cases on appeals of tax notices-decisions concerns the results of tax audits in which the transaction was recognized as unreal, so in appeal cases the question arises of proving (and vice versa, refuting) the reality of economic transactions. The available array of information on this issue needs to be organized, researched, compared and recommendations formed, in order to achieve a greater degree of legal certainty. The reality of an economic transaction is a necessary requirement for displaying the results of the transaction in the taxpayer’s tax records. The criteria of reality have been developed by judicial practice, but they need to be detailed and clarified in order to achieve legal certainty. It is the controlling body that must prove the unreality of the business transaction, however, if it provides such evidence, the tax payer has the duty to refute them. The article determines the distribution of the burden of proving the reality of economic transactions, determines the place of private law and criminal law prejudice in proving the reality or unreality of transactions. The significance of the presumption of legality of the transaction for tax legal relations is clarified, the importance of primary documents in the process of proof in such disputes is investigated, it is determined the line between the principle of individual responsibility of the payer and the doctrine of “due diligence”, recommendations are provided for supervisory authorities and taxpayers regarding proof in this category of disputes.