Abstract: We here present a new technique for the measurement of water isotope ratios in an online fashion using Cavity Ring Down spectroscopy in the Near Infra-Red region. The technique reaches a level of precision comparable or better to what can be achieved with traditional Mass Spectrometry achieving very low instrumental drifts. Attached to an ice core melter the system can offer water isotope measurements of unprecedented resolution. We demonstrate the feasibility of the technique by deploying the system on the Greenland ice sheet during the NEEM ice core project. The online nature of the data calls for a different approach regarding data analysis. We outline the methods we use in order to put the water isotope time series on a depth scale. Based on a step response analysis we are able to derive transfer functions that describe the attenuation of signals with different wavelengths. Allan variance analysis is also used to assess the precision and long term stability of the analytical system. Results are compared to discrete samples and an overall agreement is observed. The high resolution obtained with this technique can be useful for temperature reconstructions based on the study of the isotopic diffusion in firn. In this work we describe the process of water isotope firn diffusion and derive expressions for the diffusion length. Based on a diffusivity parametrization and a series of different temperature history scenarios we are able to model the diffusion length history. Spectral estimation techniques applied on the time series can yield the diffusion length value. We show how this is practically done and exemplify it by using available water isotope high resolution data from Dome C. We compare the model and data outcome and based on that we infer past isotope slopes for the last 90 kyr for the Dome C site. Our results indicate a variable isotope slope during this period. The temperature history inferred from the estimated isotope slope shows a glacial– interglacial step of approximately 9 K and warming events during Marine Isotope Stages 3 and 4 that reach Holocene levels in terms of temperature.