Orthogonal bipolar spreading sequences are used in direct sequence code division multiple acces (DS-CDMA) systems for both spectrum spreading and channel separation. The most commonly used sequences are Walsh-Hadamard sequences of lenghts being an integer power of 2. A construction based on Williamson's arrays leading to sequences of lengths N ≡ 4(mod 8) is presented in the paper. Aperiodic correlation characteristics, for example sequence sets of lengths 12-252 are presented. The correlation properties of the sequence sets are later improved using a diagonal modification technique.