We have investigated the incommensurate-commensurate lock-in phase transition in EuAs3 by the relatively new resonant X-ray magnetic scattering technique. Strong resonance enhancements of the magnetic scattering have been observed at the LII and LIII absorption edges of Eu. The intensity of the 1, 0, 5/2 magnetic reflection of the commensurate phase decreases continuously with increasing temperature and at TL = 10.3 K drops abruptly to zero. Satellite reflections characteristic of the incommensurate phase appear at T ≥ TL. By using the excellent Q-resolution of the X-ray magnetic scattering experiment we have determined the temperature variation of both x and z components of the propagation vector of the incommensurate sine-wave phase stable in the temperature range 10.3 K ≤ T ≤ 11 K. © 1998 Elsevier Science B.V. All rights reserved.