In the present study, we have determined the intrinsic refractive index of anodic aluminium oxide, which is originated by the formation of nanoporous alumina templates. Different templates have been fabricated by the conventional two-step anodization procedure in oxalic acid. Their porosities were modified by chemical wet etching allowing the tuning of their effective refractive indexes (air-filled nanopores + anodic aluminium oxide). By standard spectroscopic light transmission measurements, the effective refractive index for each different template was extracted in the VIS-NIR region. The determination of the intrinsic anodic aluminium oxide refractive index was performed by using the Maxwell-Garnett homogenization theory. The results are coincident for all the fabricated samples. The obtained refractive index (∼1.55) is quite lower (∼22%) than the commonly used AlO handbook value (∼1.75), showing that the amorphous nature of the anodic oxide structure strongly conditions its optical properties. This difference is critical for the correct design and modeling of optical plasmonic metamaterials based on anodic aluminium oxide nanoporous templates. ; The authors acknowledge the financial support from Spanish MINECO under project FIS2013-45469-C4-4-R and FCT of Portugal under projects UID/EEA/50014/2013 and UID/NAN/50024/2013, and NORTE-07-0124-FEDER-000070 –Multifunctional Nanomaterials and NORTE-07-0124-FEDER-000058, which are financed by the North Portugal Regional Operational Programme (ON.2–O Novo Norte), under the National Strategic Reference Framework (NSRF), through the European Regional Development Fund (ERDF), and by national funds, through the Portuguese funding agency, FCT. A Hierro-Rodriguez and J M Teixeira acknowledge support from FCT of Portugal (Grants SFRH/BPD/90471/2012 and SFRH/BPD/72329/2010, respectively) and POPH/FSE programs. ; Peer Reviewed