Differential scanning calorimetry (d.s.c.), dynamic mechanical thermal analysis (d.m.t.a.) and dielectric relaxation spectroscopy (DRS) have been used to study the a-relaxation in amorphous blends of poly(ether ether ketone) (PEEK) and poly(ether imide) (PEI). The composition dependence of the d.s.c. T-g is well described by the Couchman and Gordon-Taylor equations while the isothermal dielectric loss curves can be fitted by the empirical Havriliak-Negami (HN) function. The HN parameters can be represented in the form of cooperativity plots which show that they do not follow a weighted average with changing composition. The time scale for dipolar relaxation in the blends is dominated by PEI, which relaxes at a relatively slower rate compared with PEEK, while the composition dependence of the apparent activation energy shows a negative deviation from additive behaviour. It is likely that these observations are related to the interactions between the blend components which result in changes in the local molecular environment on blending.