The paper presents the results of examinations on the reciprocal relation between the isokinetic coefficient H of sampling, the quantity of measured mass concentration and the measuring usefulness of a dust sample aspired from the stream of solgas conveyed pneumatically in the horizontal pipeline. In gravimetric measurements of two-phase flows, the conditions under which aspiration is carried out, are extremely important. Available theoretical considerations and practical experiments usually concern aspiration in the vertical channels, where the manner of making measurements is universally known and it does not express any reservations. However, it is often necessary to conduct tests in the systems of horizontal conveying of solgas, where the conditions of taking a representative sample, even for the sake of the significant effect of the gravitational force, may vary from those suggested for measurements in the systems of vertical flow. The present paper also presents results of research on the selection of the isokinetic coefficient H in the case of aspiration in the horizontal channels. By means of a comparative experiment, one has found that the isokinetic coefficient, at which the results of the determined flow parameters are representative, has the optimal value of H = 1.05-1.09. The tests were conducted at the research station. In measurements the gravimetric method was applied to taking a representative sample. Dust samples were quantitatively analyzed by means of a grain-size laser analyzer. The tested dust was shale coal.