This paper presents a detailed mathematical model of DS-CDMA systems based on binary and chaotic spreading sequences in the case when interleavers are used to mitigate multipath fading in wideband channel. The interleaving techniques were well investigated in these systems in the case when flat fading and noise were present in the channel. However, a comprehensive analysis of systems with interleavers, operating in a wide-band channel, has not been done yet. Therefore, this paper presents detailed analysis and procedure of deriving the expressions for the probability of error in closed form, assuming a wideband transmission channel and in-channel diversity interleaving.