The paper is devoted to the study of the "liquidity" phenomenon from the standpoint of the economic and mathematical approach. The existing definitions are presented, as well as four criteria of liquidity and one criterion of illiquidity. Theoretical analysis of the properties of liquidity was carried out. Particular attention is paid to such transaction costs as spread and time factor. It is shown that liquidity is not an immanent category of an asset — it is a derivative of several factors characteristic of both the asset and its market environment. Based on the analysis, a microeconomic condition for emergence of liquidity phenomenon, possible methods and ways of identifying the degree of liquidity, as well as methods for assessing liquidity costs are proposed. Additionally, methods are proposed for assessing the economic benefits of liquidity and their contribution to the cost of similar and different assets. To assess the individual benefits of liquidity, the concepts of "equalizing rate" and "compensatory equivalent" were introduced, which are subsequently integrated into the methodology for assessing total and additional liquidity, expressed in relative or absolute (monetary) forms. The final part presents the classification of liquidity types, its combination with the investment value of liquid and illiquid assets, as well as an example of an end-to-end calculation of the proposed values. The appendix contains calculation formulas for assessing liquidity costs. The methodology of calculations proposed in the article can be applied when assessing the investment attractiveness of assets, choosing options for placing assets in the form of cash, securities or real estate.