Results of experimental investigation of the density of eutectic of a binary Pb-Bi system in the condensed state in a wide temperature range were given previously in [1][2][3]. The density of this alloy in the solid phase in the range from room temperature to the melting point was determined, as well as its melting temperature, the variation (jump) of density in melting, and the density of liquid lead-bismuth eutectic at temperatures up to 740 K. In [4], the upper temperature level of measurements of the density of eutectic lead-bismuth alloy in the liquid phase was raised to 970 K; finally, this level was raised in this study to 1225 K.The experiments were performed using the same method of penetrating γ-radiation and with the same samples of lead-bismuth eutectic of the same composition (44.6% by mass Pb + 55.4% by mass Bi) as those in [1][2][3]. This procedure for the investigation of the density of metals and their alloys in the condensed phase was described in [5] using bismuth as an example.Two series of measurements of the density of liquid eutectic of Pb-Bi system were performed: one series under conditions of increasing temperature and the other series under conditions of decreasing temperature. The results of these measurements at temperatures from 401 to 1225 K are given in Table 1. The temperature is given in this table by the 1990 International Temperature Scale (see, for example, [6]).The confidence error of the experimental data, which combined the systematic and random components, was calculated by the standard procedure [7] and turned out to be 0.3% at temperatures up to 1000 K and 0.4% at higher temperatures.At first, the results of each series of measurements were processed separately of each other by the least squares method. The approximating equation was provided by a polynomial of the form ,(where ρ is the density of investigated metal melt, kg/m 3 ; T is the absolute temperature, K; and a 0 and a 1 are independent parameters. The coefficients of these equations for the first and second series of experi-600 800 1000 1200 T, K -0.2 0 0.2 (ρ eq2 -ρ eq1 )/ρ eq1 , % Fig. 1. Comparison of the results of measurements of the first and second series: (1) corridor of the mean-square error of calculation data of the first series, (2) of the second series. ρ a 0 a 1 T + =