The monthly time series of the Southern Oscillation Index (SOI) is analysed to examine its chaotic characteristics. Three schemes, moving average, low-pass filter and nonlinear smoothing, were used to reduce noise and enhance chaotic properties. Autocorrelation and spectral characteristics, as well as three chaos-oriented properties -- phase space trajectory, the largest Lyapunov exponent and correlation dimension -- were then examined.No significant signs of chaotic behaviour were found for either the noise-reduced SOI time series or the raw one. Although it contains long-term periodicity, the SOI time series is considered to be stochastic rather than chaotic from the viewpoint of dynamical systems theory.