A study into the effects of different macroeconomic variables on the comovement between the Stock Exchange of Thailand (SET) Index and the Standard & Poor's 500 (S&P 500). These effects are tested using OLS regression in an annual and quarterly frequency. Real Interest Rate Difference between the two countries is found to have a significant negative effect, likely due to investors turning to Thailand when it has higher real interest rate. Income Level is found to be a significant positive effect, as countries with higher income are more integrated with the US. As the economy naturally grows, it is likely that the comovement will rise. For investors, long term risk diversification and arbitrage opportunities will be fewer, but they will still be present in the short term.