Cobalt and iron-based alloys with tendency to amorphization is a hot topic nowadays due to their unique magnetic and electrical properties. In the present work the differential thermal analysis (DTA) and density measurements of Co48Fe25Si4B19Nb4 alloy (base composition) with small additions of rare earth metals (Nd, Sm, Tb, Yb) are performed. The temperatures of phase transitions in the alloys are determined by DTA. It was found that small additions of rare earth metals (1 and 2 at %) increase solidus temperature and decrease liquidus of the base composition. Small thermal effects that were observed in the alloys with neodymium, terbium and ytterbium in the range 1080–1100°С, may indicate the decomposition of REM-B type compounds. In the alloys with samarium such effects were not found. That means that samarium does not form the Sm2B5 phase in the investigated alloys but goes into solution. Density was studied experimentally in a wide temperature range, including crystalline and liquid states, on an automated setup where the absolute variant of gamma-penetration method was realized. It is shown that temperature dependences of density in crystalline state are nonlinear and in liquid state are well described by linear functions. When melts are overheated above a certain temperature, density hysteresis (incoincidence of heating and cooling curves) appears; that may indicate structural transformations occurring in the melt. It is found that the rare-earth additions increase density of investigated alloys, however, this occurs non-linearly and depends on REM content. Basing on density experimental data, the coefficients of volume expansion and molar volumes are calculated.