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[en] Positron lifetime spectra arising from micellar solutions of sodium dodecylsulphate (SDS) are interpreted in terms of a classical positronium diffusion model published earlier. Unlike the generally accepted assumptions, this model results in a non-exponential ortho-positronium (o-Ps) lifetime density function. A new method is presented for the simultaneous fitting of this lifetime density function to independent lifetime spectra recorded under the same experimental conditions. Among the fit parameters Dsub(p), the diffusion coefficient of o-Ps in the solvent (i.e. heavy water) phase is studied in detail; a detailed error analysis for Dsub(p) is also given. Provided that the mean aggregation number of SDS micelles is about 60, the published Dsub(p) values show that the diffusion coefficient of o-Ps in (heavy) water at room temperature is lower than that of small ions and molecules and the Arrhenius plot indicates a strong o-Ps localization in the solvent. The hydrodynamic radius of o-Ps is calculated from the o-Ps and micellar diffusion coefficients and from the micellar radius; it is greater than that of small ions and molecules and this can be considered as an independent indirect proof for the existence of o-Ps bubble in the (heavy) water. (author)