Analysis of 35 PAH (parental and alkylated homologues) was carried out in 11 marine surface sediment samples collected along a central transect in the biggest Galician ría. The samples were collected using a box-core dredge and, after freeze-drying, were kept frozen until analysis. The sediments were then extracted by ASE (Acceleretad Solvent Extraction) and the quantification of PAHs was performed using gas chromatography coupled to mass spectrometry (GC-MS) with the aid of deuterated PAH internal standards. The total concentration (Σ35 PAH) measured in all sediment samples ranged from 44.8 to 7901 ng/g dw, the highest value was found in sediments collected near the harbour and the cleanest positions were found in the outer zone of the ría. To investigate the origin of PAHs, three diferent diagnostic ratios based on isomeric relations were used: Ant/(Ant+Phe), IP/(IP+BghiPer) and BaA/(BaA+Chry) ratios were plotted against Fla/(Fla+Pyr) and a pyrolitic origin was pointed out as the main source. In addition, the relative abundance of alkylated PAHs is useful for source distinction as the presence of higher concentration of alkyl homologues than parent compounds indicates a petroleum source. In this study Phe, Pyr, Chry and PAH alkylated series were considered. Values smaller than 0.4, 0.5, 1 and 0.8 respectively are associated with exclusively pyrolitic sources and values bigger than 2.5, 0.9, 2.5 and 2.5 are indicative of petrogenic sources. As concluded before, most samples showed a mixed or pyrolitic source and only some ratios (i.e. MChry/Chry) can identify petrogenic evidences in some locations. PAHs can also be formed during diagenetic processes for what perylene has been described as a good indicator. The ratio Per/penta-aromatic isomer also indicates a pirolityc origen of this compound for most of the locations. Other variables than PAHs, such as grain size distribution and COT were also determined. Values vary from 2.3 to 86.5% of particles <63um and from 0.5 to 3.5% for COT. No clear relationship between these variables and the PAH concentrations was found. Two main conclusions can be drawn, first that a gradient can be observed as sediments get cleaner from the river Ulla mouth to the outer zone of the ría and second that most samples showed a mixed or pyrolitic source and only some locations show a less pyrolitic origin, with some diagenetic contributions.