Faced with intense competition in the market, commercial banks have engaged in aggressive marketing. This entails increased annual allocation for marketing promotion and competitive pricing of services. However, with reports indicating that less than 50% of Kenyans operate bank accounts; this is a likely indicator that promotional mix choices by commercial banks have not been very effective. This study sought to identify the factors that affect the choice of promotional mix decisions by commercial banks in Kenya. The study was based on the contingency theory. It adopted a case study approach. The marketing departments of KCB branches constituted the unit of analysis for the study. Stratified and random sampling techniques were used to select 118 branches from the 170 branches of KCB. This study relied on both primary and secondary data. Primary data was collected through questionnaires while secondary data was sourced through review of relevant records. The data analysis for the study was conducted through qualitative and quantitative methods. Regression analysis was used to analyze the degree of relationship between promotional mix decisions adopted by the bank and the factors that inform such decisions. The analyzed data was presented using frequency tables, proportions and percentages. The study findings revealed that availability of funds is the most important factor affecting the choice of promotional mix by commercial banks, with 60% of the respondents citing it as a very important factor affecting the choice of their promotional mix. The study also established that availability of funds has a strong positive relationship (r=0.745) with cost of promotion at 5% level of significance. The study recommended that the heads of marketing at the branches of commercial banks should consider the funds available to them before designing their promotional mix, and mainly use personal selling in their promotional campaigns. Additionally, the marketing staff at the branches should be regularly trained to improve their ability to design effective promotional mix. Study findings will provide the policy makers of commercial banks with a framework for making promotional mix decisions. The findings will also stimulate further academic research in the field of marketing promotion by commercial banks.