Accurate definition and measurement of the thermodynamic dead-volume, V m in liquid chromatography is essential for the correct evaluation of capacity ratios. Many different recipies for determining V m have been suggested. We propose that V m be defined as the total volume of all eluent components within the column bed. It is shown that V m so defined, is given by V m V A* x A + V B* x B + … where V A* etc. are the elution volumes of isotopically labelled eluent components A etc., and x A etc. are the volume fractions of A etc. in the eluent fed to the column. For an ( N+ 1) component eluent there will be ( N+ 1) such peaks. If a mixture of same eluent components but with different composition is injected into the column, N solvent disturbance peaks will be obtained which, in general, will not coincide with the peaks for labelled eluent components. The cases of binary and ternary mixtures are examined in detail and the transition from peaks due to trace components into solvent disturbance peaks is explored and clarified. The treatment is generalised to ( N+ 1) component mixtures and leads to important results relating to vacancy chromatography. Experimental data are presented for binary and ternary eluents which provide practical validation of the above equation. For binary eluents, A + B, the same data allow calculation of partition isotherms for A and B between bulk eluent and space within the column bed, while the elution volumes of the solvent disturbance peaks in A + B give the gradient of the isotherm. This theoretical connection is accurately confirmed by our experimental data and by that of previous investigators [R.M. McCormick and B.L. Karger, Anal. Chem., 52 (1980) 2249]. On the basis of our theory and experimental data, a critique is given of the various methods currently claimed to give values for V m.