In Taiwan Southern Min, a syllable's underlying tone is only realized in phrase-final positions, whereas in non-boundary positions, it is realized as another tonal category due to the application of sandhi rules. Previous studies found that the realizations of sandhi tone (e.g., high-falling becomes high-level) are indistinguishable from those of surface base tone (e.g., high-level realized as high-level), suggesting tonal neutralization. However, given that past investigation utilizes mainly laboratory speech, and that with spontaneous speech data, the control for confounding factors is insufficient, it remains unknown whether neutralization is genuine, and whether neturalization interacts with speech register. The goal of the present study is to examine sandhi tone realizations in spontaneous speech in greater detail. We analyzed the entire F0 contours of monosyllabic words with Generalized Additive Models. Specifically, for every tonal category, we compared the realizations of sandhi tone with their respective base tones. Results showed that sandhi and base tones differ in not only pitch height but also contour shape, though to a varying degree depending on gender and tonal category. The current finding provides evidnece of incomplete neutralization in spontaneous speech, hence posing an interesting question to the phonological status of tone sandhi in Taiwan Southern Min.