Covalent attachment of poly(ethylene glycol) (PEG) to therapeutic antibody fragments has been found effective in prolonging the half-life of the protein molecule in vivo. In this study analytical ultracentrifugation (AUC) in combination with small angle X-ray scattering (SAXS) has been applied to a number of antibody fragments and to their respective PEGylated conjugates. Despite the large increase in molecular weight due to the attachment of a 20-40 kDa PEG moiety, the PEGylated conjugates have smaller sedimentation coefficients, s, than their parent antibody fragments, due to a significant increase in frictional ratio f/f o (from $1.3 to 2.3-2.8): the solution hydrodynamic properties of the conjugates are clearly dominated by the PEG moiety ( f/f o $3.0). This observation is reinforced by SAXS data at high values of r (separation of scattering centres within a particle) that appear dominated by the PEG part of the complex. By contrast, SAXS data at low values of r suggest that there are no significant conformational changes of the protein moiety itself after PEGylation The location of the PEGylation site within the conjugate was identified, and found to be consistent with expectation from the conjugation chemistry.