This paper reports on a preliminary investigation of the sound field properties inside a large Roman Catholic Church in Barcelona, the Sagrada Familia Basilica, which is a World Heritage Site although its construction has not been completed. The impulse responses were measured at 5 sound source positions combined with 14 measurement locations inside the Sagrada Familia Basilica, and the Impulse response to Noise Ratios (INR) were examined to check the reliability of the measured impulse responses. The room acoustic parameters were calculated and the following 5 sound field properties in the Sagrada Familia Basilica were analysed: reverberation, spaciousness, loudness, warmth and clarity. No optimal values of room acoustic parameters for such large volume churches have been found in the literature; thus, the preferred values of the reverberation time (T20) and the Early Decay Time (EDT) for small volume churches, and the preferred values of the middle frequency strength of sound (Gmid), the low frequency strength of sound (G125), the clarity (C80) and the binaural quality index (1 - IACCe) for concert halls were compared with the measurement results to illustrate a primary impression of the listening experience in the Sagrada Familia Basilica. The reverberation time (T20) and the EDT in the Sagrada Familia Basilica are much higher than the preferred values, while the middle frequency strength of sound (Gmid), the low frequency strength of sound (G125) and the clarity (C80) are less than the preferred values. The binaural quality index (1 - IACCE) is just above the typical satisfactory value
