The effect of positive thermodiffusion of colloidal particles under convection of magnetic fluids in connected vertical channels of 3.2 × 3.2 mm 2 square cross-section and height 50 mm heated from below is analyzed. Below the critical Rayleigh number, particle thermophoresis in vertical generates unstable density stratification in fluid at rest. This leads to rapid bursts (~1 min) of concentration convection arising periodically (~4 h). Under developed convection, above the critical Rayleigh number particle thermophoresis in horizontal direction generates concentration inhomogeneities in the neighborhood of the channel walls and provokes convective flow instability that leads to the periodic change (~1 h) in the direction of convective stream. The reasons of the oscillatory instability of mechanical equilibrium observed experimentally at positive sign of the Soret coefficient are discussed.