Abstract: Both bilayer and multilayer samples are bombarded at room temperature with Xe~+ (300 keV)or Ar~+ (90--120 keV) for the doses of 1×10~(15)--1×10~(17)cm~(-2). AES, XPS and RBS are used forthe depth profiling of compositions. TEM observations are also carried out on multilayer samples.It is found that ion beam mixing occurres in cascade regime Q∝Φ~(1/2) for the multilayers and effec-tive Dt∝Φ~(1/2) for the bilayers, respectively. Nonuniform mixing is observed in multilayers, partic-ularly with Xe~+ bombardment, which is consistent with the calculations of the deposited energydensity distribution. Metastable β-Ti-Mo solid solution of bcc structure is formed after mixing.Impurities such as C, O significantly suppress the mixing and are also redistributed buring ionbombardment.