Optimum fluid placement is crucial for successful acid stimulation treatments of long horizontal wells where there is a broad variation of reservoir properties along the wellbore. Various methods have been developed and applied in the field to determine acid placement and the effectiveness of diversion process, but determining the injection profile during a course of matrix acidizing still remains as a challenge. Recently distributed temperature sensing technology (DTS) has enabled us to observe dynamic temperature profiles along a horizontal wellbore during acid treatments. Quantitative interpretation of dynamic temperature data can provide us with an invaluable tool to assess the effectiveness of the treatment as well as optimize the treatment through on-the-fly modification of the treatment parameters such as volume, injection rate and diversion method. In this study we first discuss how fluid placement can be quantified using dynamic temperature data. A mathematical model has been developed to simulate the temperature behavior along horizontal wellbores during and shortly after acid treatments. This model couples a wellbore and a near-wellbore thermal model considering the effect of both mass and heat transfer between the wellbore and the formation. The model accounts for all significant thermal processes involved during a treatment, including heat of reaction, conduction, convection. Then a fast and reliable inversion procedure is used to interpret the acid distribution profiles from the measured temperature profiles. We extend the real-time monitoring and evaluation of the acid stimulation treatment in horizontal wells to calculate the evolving skin factor as a function of time and location along the wellbore. As the skin factor is a reflection of the injectivity, it will indicate directly if the acid stimulation is effective and if diversion is successful. The approach to monitor the evolving skin along the lateral is to use a proper pressure transient model to calculate skin factor by integrating the inversion results of the temperature data (acid injection profile) with either surface or bottomhole injection pressure. This method can help engineers to optimize an acid stimulation in the field.