[en] Precise relative measurements of the velocity of first sound in solutions containing 0.05, 0.155 0.51% ³He have been made in the temperature range 0.1--0.7⁰ K. It was found that the nature of the temperature dependence of the velocity changes with changing ³He concentration. A maximum velocity (the magnitude of which increases with decreasing ³He concentration) is found in very weak solutions, while the velocity falls monotonically with increasing temperature in more concentrated solutions. The dependences obtained cannot be explained within the framework of existing theoretical concepts